DANBURY, Conn. - Overweight and out of shape, Union Carbide Corp. is beginning to implement a long overdue restructuring of its operations, Wall Street analysts say.

In a series of moves in recent weeks, the nation's third largest chemical company has started to trim some fat from its multibillion-dollar operation.

Its bylaws and pension plan have been modified and its corporate structure simplified, and, by early next year, about 4,000 white-collar workers with hefty paychecks and a number of non-strategic businesses will be shed.

Carbide said it hopes to save $250 million, or an average of $62,500 in salary and benefits per employee. It is hoping to raise $500 million through divestitures.

Plans also call for $500 million to be taken from an employee pension plan to buy back up to 10 million shares of stock.

Finally, an unprecedented $990 million charge will be taken against 1985 earnings, and depreciation periods will be shortened. The charge, which the company said would reduce after-tax 1985 earnings by $8 per share, is believed to be the largest in the chemical industry's history.

"This is a short-term program," Carbide Chairman Warren M. Anderson said. If the moves do not achieve the company's goal of increased profitability in 1986, he said, "We'll cut some more."

Since the seven-point program aimed at improving Carbide's financial position and attractiveness to investors was announced on Aug. 28, the company has repeatedly said it was part of a continuing restructuring begun in the mid-1970s.

Analysts, however, note that at no time in Union Carbide's history have such dramatic moves come in such rapid succession.

"Anyone who believes this is part of something that began 10 years ago probably believes in the Easter Bunny and Santa Claus, too," said Paul T. Leming, an analyst at Kidder Peabody & Co.

Nevertheless, he and others on Wall Street welcomed the actions as addressing problems at Carbide that some believe have their roots in the 1970s.

Singled out most frequently by analysts as poor decisions by the company are the 1978 sale of Carbide's former New York City corporate skyscraper during a weak city real estate market, its reluctance to halt capital spending in the early 1980s, and a heavy reliance on the struggling U.S. steel industry to support its once highly profitable metals and carbons segment.

"They have been very slow to react to the fact that their businesses were in terrible shape. The company's been asleep. The fat had built up in this company over a number of years," said Leslie C. Ravitz of Solomon Brothers.

"Do these steps solve all of the company's problems?" he asked. "No, but it certainly enhances the outlook for 1986."

Most analysts believe the recent moves are unquestionably a response to toxic gas leaks that have thrust the normally low-profile company into the international spotlight, eroding its stock price along with public confidence in its ability to operate safely and profitably.

On Dec. 3 , 1984, a chemical leak from a Union Carbide factory killed about 2,000 people in Bhopal , India. A little more than eight months later, on Aug. 11, another leak injured 135 people in Institute, W.Va. Smaller leaks also have been reported at the company's West Virginia plants.

On top of the leaks, GAF Corp. surfaced with a potential takeover threat in August.

Publicly, Union Carbide has said the accidents and GAF's accumulation of Carbide stock have played no role in the retrenchment. However, in a recent videotaped message to Carbide employees, Anderson warned that "there are outsiders whose stock in trade is imposing change on companies lacking the will or desire to move boldly on their own."

In a meeting with securities analysts on Sept. 4, Anderson acknowledged he had talked to GAF, but he said "nothing specific" was discussed. He said GAF had not proposed a merger, which GAF said might be a possibility when it filed notice with the Securities and Exchange Commission that it had purchased more than 5 percent of Union Carbide's stock.

GAF now owns 9.9 percent of Carbide's stock and is seeking federal permission to buy as much as 15 percent of Carbide's 70.4 million outstanding shares "for investment purposes."

Samuel J. Heyman, GAF's chairman, took control of his company in a proxy fight with only 4.9 percent of its stock. Some analysts have speculated that Heyman intends to attempt a Carbide takeover, while some think he has his eye on trading the stock for Carbide's specialty chemicals businesses.

Regardless, most think Carbide has been put on the defensive.

"Obviously, these are defensive moves," said James M. Arenson of Donaldson, Lufkin Jenrette Securities. "To avert a takeover, they bit the bullet on everything at one time."

Union Carbide, with 1984 revenues of $9.5 billion, is about 13 times the size of GAF. Based in Wayne, N.J., GAF's major products are specialty chemicals and building materials. Chemicals produced about 85 percent of the company's $41 million profit in 1984.

Union Carbide reported net income of $323 million, or $4.84 per share in 1984. Only three years earlier, it earned $649 million, or $9.56 per share.

Union Carbide said its new financial program at completion should save some $300 million a year, increase earnings by $2 per share over the long term and raise return on equity from 1984's 6.6 percent - about the lowest in the industry - to at least 10 percent.

Spokesman Tom Failla said the company's return-on-equity goal, within an unspecified time frame, is between 15 percent and 16 percent. Major competitors of Union Carbide, such as Monsanto Co. and Dow Chemical Co., should have returns on equity at the end of this year of between 10 percent and 10.5 percent, analysts said.

Anderson said that in 1986 "if we haven't cut enough in this go-around, we'll cut some more. If we haven't jettisoned some businesses we should have, we'll jettison some more."

Since 1978, Union Carbide has gradually attempted to reposition itself, selling its namesake calcium carbide operations and de-emphasizing petrochemicals, metals and carbons.

It has concentrated on industrial gases, specialty technology services and consumer products, areas the company sees as high in growth and profit potential. Some of Union Carbide's well-known lines are Prestone antifreeze, Glad trash bags, Eveready batteries and Simoniz car-care products.

This summer, it sold its welding and cutting business for $57 million and it cryogenic equipment operation for an undisclosed price. It also purchased STP from Beatrice Cos. Inc. for $87 million and bought the biocide business of a French company for an undisclosed price.

In July, Carbide reorganized management by creating a vice chairman's slot to be filled by former president Alec Flamm and setting up two divisions - one to oversee chemicals and plastics, the other to focus on consumer, industrial products and services.

This month, Carbide created and filled the new post of vice president in charge of community and employee health, safety and environment to report directly to Anderson. The company also replaced the manager of its Institute chemical plant.

"I'm going to clean up the shop. It's going to happen before I leave," said Anderson, who at age 63 has about 18 months to go before retiring.