The Reagan administration's plan to sell China $550 million in avionics to upgrade its most advanced interceptor aircraft has sparked opposition from congressional supporters of Taiwan and threatens to upset the delicate political balance that Washington has maintained with the rival Chinese governments for the past three years.

The proposed sale is seen as an important test of the two-pronged U.S. policy of forging military relations with the communist government in Peking while guaranteeing the defense capability of the capitalist government on Taiwan.

The avionics package under consideration is the most costly and sophisticated equipment that Washington has offered Peking in their evolving military relationship. It involves the sale of U.S. electronics to modernize the navigation, fire control and communications systems of 50 Chinese F8 jet fighters.

Defense Department officials said the deal, which will be submitted to Congress next month, is intended to improve China's air defenses against a possible Soviet attack and to signify a new level of military cooperation between Peking and Washington.

But China specialists fear that the plan could backfire by reopening the most contentious issue in Sino-American relations: U.S. sales of high-performance aircraft to Taiwan.

Taiwan and its U.S. backers have begun pressing the administration to upgrade the island nation's F5E jet fighters, contending that improved Chinese F8s would erode Taiwan's traditional air superiority and imperil the stronghold where nationalist Chinese fled after losing control of the mainland to the communists in 1949.

"I don't like the idea of helping the Red Chinese without helping the Taiwanese," said Sen. Barry Goldwater (R-Ariz.), chairman of the Senate Armed Services Committee.

Goldwater and Rep. Gerald B. Solomon (R-N.Y.), a member of the House Asian and Pacific affairs subcommittee, have urged the administration to replace Taiwan's F5Es with more advanced F20 jet fighters. Sen. Frank H. Murkowski (R-Alaska), chairman of the Senate's East Asian and Pacific affairs subcommittee, wants to equip Taiwan's F5Es with all-weather capability to match any F8 improvements.

Peking bitterly opposes U.S. arms sales to Taiwan, which it regards as a breakaway province of China, and drove Sino-American relations to the breaking point in the early 1980s when the administration was considering proposals to sell Taiwan a new combat fighter.

Harry Harding, a sinologist at the Brookings Institution, said the aviation electronics, or avionics, proposal that has rallied Taiwan's supporters "runs the risk of complicating our lives on the most sensitive issue with China."

"What this does is raise the danger of rocking the boat by suddenly reintroducing the issue of an advanced figher to Taiwan," Harding said.

The administration defused the arms sales issue in August 1982 by agreeing in a communique to freeze the quantity and quality of its weapons supplies to Taiwan. Peking, in turn, promised to pursue a peaceful course in attempting to reincorporate the island.

At the same time, the administration renewed its pledge to observe a 1979 statute requiring the United States to provide Taiwan with "sufficient" arms to maintain an adequate self-defense capability.

For the past three years, these commitments have set the framework for stable U.S. relations with Taipei and Peking. The administration has fostered longstanding U.S. commercial ties with Taiwan and plans to sell it $740 million in arms in fiscal 1987, about $100 million less than the level prior to the 1982 agreement.

In line with its philosophy that a strong China deters Soviet aggression in Asia, the administration also resumed a strategic dialogue with Peking and has sold it $150 million in helicopters with military engines and $45 million in equipment for a large-caliber artillery ammunition plant.

A Pentagon official said the proposed avionics deal presents no threat to Taiwan because the F8 is designed for the defensive mission of shooting down invading bombers.

"No Chinese military officer in his right mind would deploy it against Taiwan," he said of the F8. "It's just not designed to do combat."

He noted, moreover, that the communist government has sought since 1979 to peacefully reunify Taiwan while turning to the West for capital and expertise to modernize China. He said Peking realizes that "the first time it takes off and heads for Taiwan, the door to the West is shut."

The official said congressional proposals to sell Taiwan the F20 or upgrade the F5E flout the 1982 agreement and would not be approved by the administration.

Taipei's backers in Congress believe the 1979 law requiring U.S. arms to guarantee Taiwan's self-defense capability takes precedence over the 1982 communique and justifies their proposals for new and better aircraft sales to the island.

Solomon said the avionics sale would blunt Taiwan's qualitative edge in aircraft and "make it impossible for them to ward off attack" by the much larger communist air force.

"There's no way Taiwan could protect itself," he said.

Murkowski, noting that an F8 improved with all-weather capability would raise the "potential of threat" to Taiwan, said the United States is obligated under the 1979 law to provide the island "with deterrent capability equal to the threat."

Martin Lasater, director of the Asian Study Center at the Heritage Foundation, said that Taiwan's 19 million people would be more "psychologically vulnerable" to communist threats if the island lost its tradtional air superiority.

"Military balance reflects not only hardware but psychology," he said.

Congress will be formally notified of the sale April 8, according to a Defense Department official. The deal will be automatically approved if no action is taken in 30 days of notification. A two-thirds majority of both chambers is required for disapproval.