UPPER SADDLE RIVER, N.J. (AP) _ Western Union Corp. unveiled a major financial restructuring plan Wednesday aimed at ending the company's severe cash squeeze and a three-year streak of losses.

The four-part program is designed in large part to reduce the amount of cash needed to service the debt of the telecommunications concern, whose problems have forced it to omit stock dividends for six straight quarters.

Robert S. Leventhal, Western Union's chairman and chief executive, said the plan includes an offer to exchange new debt securities and common stock for almost all the corporation's debentures and notes currently outstanding.

Other elements of the restructuring to be completed during the next four months are:

-A plan by the investment firm Drexel Burnham Lambert Inc. to raise up to $200 million through the sale of new debt securities to meet Western Union's obligations to banks and provide more working capital.

-New agreements with its banks to pay $300 million in bank debt over five years if at least $120 million of the debt is paid by June 30.

-A merger of Western Union into its principal subsidiary, Western Union Telegraph Co.

Western Union lost $367.2 million in 1985, $58.4 million in 1984 and $59.1 million in 1983.

The losses reflected a variety of factors, including escalating costs and the company's heavy start-up investment in Easylink, an electronic-mai l service Western Union now considers a flagship product.

In addition, Western Union has taken charges, totaling in the hundreds of millions of dollars, against operating earnings to reflect the writedown in the value of older communications equipment.

Analysts described the company's restructuring program as innovative but said it would not resolve all the company's financial woes.

The proposed changes also were not well-received on Wall Street. After the company's announcement, Western Union's common stock dropped $1.12 1/2 a share to $7.87 1/2 in New York Stock Exchange composite trading.

Harry Rosenthal, who follows the company for the investment firm Bear, Stearns & Co., said the plan was ''intelligent and well thought out.'' But he said a weakness was that the $200 million was still to be raised.

However, Western Union said Drexel Burnham was ''highly confident'' it could raise the funds if the other restructuring steps were completed.

Leventhal said the exchange of old debt securities for new debt and common stock would ''significantly reduce'' Western Union's debt-service requirements by extending the maturities of its debt, deferring cash contributions it needs to retire the debt and giving the company the option to pay debt interest with shares of common stock.

Western Union said holders of at least two-thirds of each of its current debt issues must approve the changes.

The company also said that under its new bank agreements, once the overall restructuring plan is completed, and if at least $120 million of the bank debt is paid by June 30, a five-year repayment schedule would take effect for the remaining debt. The existing bank agreements also would be extended from March 31 until June 30 or until the restructuring is completed, whichever is sooner.

Meanwhile, the plan to merge Western Union into Western Union Telegraph would allow the company to use stock to pay the $37.5 million in previously deferred dividends it owes preferred stockholders, the company said.

The merger also would permit the combined company to streamline operations and reduce operating costs, it said. The merger is subject to stockholder approval at the companies' annual meetings scheduled for May 15.

Rosenthal said the merger between the corporation and the subsidiary, which has more assets, was designed to place creditors of both companies on equal footing.

He said that while the plan's success depends on several pieces falling into place, the program would ''give the company a chance of carrying on and surviving.''

Glenn Pafumi of Dean Witter Reynolds Inc. agreed that the plan would allow Western Union a chance to devote more attention to ''getting the revenues back on track.''