President Carter yesterday approved most-favored-nation tariff treatment for the People's Republic of China, marking a major advance in U.S. relations with Peking and raising serious questions about U.S. relations with its arch-rival, the Soviet Union.

The long-expected decision, which must be approved by both houses of Congress, was described by Carter as intended to strengthen both economic and political ties with China.

Carter said nothing in a statement to Congress about repercussions involving the Soviets, but officals said the administration continues to hope that a parallel action to provide trade advantages for Moscow can be taken in the coming months.

Hopes aside, yesterday's announcement marks an important departure from the stated policy of evenhanded treatment of the rival giants of international communism. In view of greater support for Peking on Capitol Hill and greater problems for Moscow in complying with congressionally imposed conditions, action to give the Russians similar benefits is far from assured.

Backed up by public statements of the president, Secretary of State Cyrus R. Vance had argued against "a skewed foreign policy" that could generate serious consequences.

The uneven result was brought about, according to officials, by three essentially unrelated developments:

Swift development of Washington-Peking relations, encouraged by presidential assistant Zbigniew Brzexinski and facilitated by Chinese flexibility on the economic and emigration issues involved. The U.S.-China trade pact during Secretary of Commerce Juanita Kreps' visit to Peking in May led almost inevitably to approval of most-favored-nation tariff treatment within a few months.

Soviet refusal to provide any written assurances or even verbal acknowledgement of compliance with the 1974 Jackson-Vanik amendment, which links most-favored-nation status for communist countries to their policy on emigration of their citizens. Although Jewish emigration from the U.S.S.R. is up to near-record levels, the Soviets adamantly refused in diplomatic talks this spring and at the June summit in Vienna to provide assurances which Carter could supply to Congress, on grounds that this would be interference in their internal affairs.

The judgment by U.S. Senate leaders in recent weeks that a controversial proposal to grant favored tariff status to Moscow at this time would be a new impediment to ratification of the endangered U.S.-Soviet treaty on strategic arms. The Senate leaders are reported to be open-minded about trade benefits for the Soviets if and when the SALT II treaty is ratified, but no firm decision has been reached on such a proposal in either the executive or legislative branch.

The Soviets had been informed in advance through Ambassador Anatoliy F. Dobrynin of the action taken yesterday. There was no immediate reaction from the Kremlin.

Most-favored-nation status under the trade laws will allow China to qualify for tariff reductions negotiated with any country by the United States since 1934. In many cases these are very large reductions.

At the Commerce Department, where Chinese Foreign Trade Minister Li Qiang appeared with Secretary Kreps to mark the announcement, officials estimated that the new arrangements will make possible expansion of bilateral trade to $5 billion yearly by 1985. Two-way trade with Peking reached $1.2 billion last year and should be double that this year, according to officials.

Carter, in a letter to both houses of Congress, urged the lawmakers to act "as soon as possible" to approve the trade agreement signed with Peking and accept the emigration assurances he supplied. Carter reported "a marked relaxation" of Chinese emigration policy and a dramatic jump in the number of emigrants.

Chinese Vice Premier Deng Xiaoping at the White House last February offered to "send you 10 million Chinese tomorrow" to meet the emigration requirements. Carter quickly declined.

On Capital Hill, Sen. Henry M. Jackson (D-Wash.) hailed Carter's decision as the most important development in U.S.-China relations since normalization of diplomatic relations last January.

Rep. Charles A. Vanik (D-Ohio), co-author of the Jackson-Vanik amendment, said he hopes for parallel administration action regarding the Soviet Union before Congress approves the trade benefits for China. Congress has 60 legislative days to consider yesterday's action.