Sen. Adlai E. Stevenson (D-Ill.), chairman of the international finance subcommittee of the Senate Banking Committee, plans to introduce legislation Monday that would significantly alter the "Jackson-Vanik amendment" restricting trade benefits to the Soviet Union and -- potentially -- China.

Stevenson's move is the first concrete step in what is likely to be extensive maneuvering this year to give China most-favored-nation tariff status and Export-Import Bank credits as part of normalization of relations with Peking.

The Carter administration is anxious to take these steps, but only if they can be taken more or less simultaneously with the Soviet Union. Administration officials fear that relations with the Soviets would be severely strained if China appeared to be getting U.S. advantages that Moscow was denied.

Stevenson's proposed legislation would address this problem by setting up new methods for granting the benefits to communist countries.

Under the Jackson-Vanik amendment, named for Sen. Henry M. Jackson (D-Wash.) and Rep. Charles A. Vanik (D-Ohio), trade benefits cannot be granted to communist countries that restrict emigration of their citizens who want to live elsewhere. This restriction can be waived by the president, with Congress' approval, if the government in question gives credible assurances that it has stopped restricting emigration.

Stevenson's measure would change that, giving the president the authority to grant trade benefits if he determines that doing so would "lead substantially to the achievement of the free emigration objectives" of the original Jackson-Vanik amendment. Congress would retain the right to veto a presidential finding of this kind, but only once every five years.

Stevenson's measure is drafted in a way that would leave the objective of Jackson-Vanik on the books, but would provide an opportunity to lift trade restrictions on the Soviet Union without requiring formal assurances from the Soviets.

The Chinese claim not to be restricting emigration now, and this contention is apparently accepted by the Carter administration and key members of Congress. Thus Jackson-Vanik would not pose an obstacle to trade benefits for the Chinese.

The Soviets have recently allowed a sharp increase in the rate of Jewish emigration from the Soviet Union. In December nearly 5,000 people were allowed to leave. Rep. Vanik has said he thinks this rate of emigration entitles the Soviets to trade benefits under Jackson-Vanik. Jackson has not publicly agreed, however.

Though Stevenson hopes to help the administration achieve its objective of equal treatment for both communist powers, the administration has not endorsed his legislation. "It's premature," one official said last night. "We want to consult with members of Congress" before deciding on a policy recommendation, the official said.

Stevenson's bill would also lift the $300 million limit on Export-Import Bank credits to the Soviet Union, replacing it with a $2 billion limit on credits to any one communist country. The $300 million limit was imposed in 1974 on Stevenson's own recommendation, at the same time Jackson-Vanik was passed.