The State Department expressed confidence yesterday that the Soviet Union will not veto the U.S. proposal for United Nations economic sanctions against Iran.

While saying that no specific assurances have been received from the Soviets, spokesman David Passage said, "We are confident there won't be a veto. They know full well what it means to us."

At the same time, other State Department officials expressed the belief that the People's Republic of China, which is Moscow's arch rival, will be more supportive of the U.S. effort than previously expected.

Despite earlier reports that China might abstain from voting on the economic sanctions, proposed by the United States, the officials said Peking has given every indication of wishing to be helpful and might vote for sanctions. The Chinese representative to the United Nations, Chen Zhu, currently is serving as chairman of the Security Council and is expected to be supportive in behind-the-scenes negotiations.

Moscow and, to a lesser extent, Peking have their own stakes in relations with Iran. But both the giants of world communism have been informed that the United States places paramount importance on international action to secure the release of the diplomatic personnel held hostage in Tehran.

Secretary of State Cyrus R. Vance raised the possibility of U.N-imposed sanctions on 2ran with Soviet Ambasador Anatoliy F. Dobrynin Dec. 6, just before the Soviet envoy returned to Moscow for consultations from which he has not yet returned. And U.S. Ambassador Thomas J. Watson Jr. took up the matter in a Moscow Meeting Friday with Soviet Foreign Minister Andrei A. Gromyko.

Gromyko is understood to have given a relatively positive response to Watson, while not supplying a definite commitment about the Soviet stand. Earlier, Soviet diplomats and press commentators were critical of economic sanctions. However, the Soviets have had little to say in public since President Carter's announcement Friday that the United States will take its case for sanctions to the U. N. Security Council.

As permanent members of the Secutity Council, both Moscow and Peking have veto power over decisions of that body. Representatives of both the communist powers joined in the unanimous Security Council resolution Dec. 4 calling on Iran to release the hostages. In addition, P.D. Morozov, a Soviet judge, joined in the International Court of Justice's unanimous order Dec. 15 for the hostages to be released.

Punitive economic action against Iran would be a major step beyond mere disapproval of hostage-taking and appeals for their release. In view of the worldwide significance of steps toward an economic embargo -- and the strong opposition of the Iranian authorities -- the sanctions plan presents difficult choices for the communist powers as well as may nonaligned nations of the Third World.

The Soviet Union, in the eyes of American officials, has taken an ambiguous attitiude toward the U.S. crisis with Iran, in keeping with the multifaceted nature of Moscow's interests.

At the United Nations and the world court and in a few comments in Soviet media, the Soviets have supported the principle of diplomatic immunity and, in effect, backed the U.S. position. U.S. officials believe this is because of the Soviets interest in maintaining their own diplomatic missions, which contain large numbers of intelligence personnel, and because of the Soviets interest in relations with the other nuclear superpower, the United States.

Several U.S. senators said Friday there would be no hope for retification of the strategic arms limitation treaty (SALT II) if Moscow vetoed Washington's call for sanctions against Iran. Any such veto, it is clear, would sour U.S.-Soviet relations on a broad front.

While giving muted support to the principles of diplomatic immunity, Moscow at the same time has launched vitriolic attacks on past U.S. relations with the deposed shah and issued daily blasts at U.S. "threats" of military action against Iran. Soviet propaganda organs have expressed solidarity with Iranian grievanes against the United States, fanned anti-Americanism and made almost no mention of the hostages.

Inside Iran, the Soviet-supported Tudeh party was the only left-wing party to favor the new constitution, which gives lifetime power to Ayatollah Ruhollah Khomeini. In return, Tudeh is the only left-wing party permitted to publish its own newspaper. Although third or fourth in size among leftist groups in Iran, the well-organized Tudeh party seeks to put itself in a position to bid for power if the Islamic revolution slides into chaos.

Surveying this mixed picture, an American official commented that so far the Soviets have sought to cover every bet in a complicated situation -- backing the United States on diplomatic immunity, backing Iran on its grievances, maneuvering the Tudeh party into place, and positioning themselves to reap benefits in Iran and the ragion should Washington take military action.

China's position on the Iranian crisis has been less complex, in view of Peking's less important stakes in Tehran.

China has modest economic ties with Iran, including an agreement reached in mid-November to purchase Iranian cotton and dried fruits. But the most important Chineses interest, as seen by U.S. officials, is to maintain political ties to compete with Moscow. Peking's relations with the Islamic revolution were damaged at the outset by Party Chairman Hua Guo-feng's visit to the shah several months before the monarch's downfall.

With U.S. Defense Secretary Harold Brown scheduled to visit Peking early in Januray to inaugurate closer Sino-American military relations, China may be particularly anxious to stay in Washington's good graces now. Nevertheless, the Chinese also are reported to be anxious no to be out of step with Third World representatives at the United Nations.

In view of China's history of harassing foreign diplomats in periods of turmoil, Peking has been surprisingly supportive of the U.S. position. A Chinese Foreign Ministry statement Nov. 26 backed the "principles of international law and diplomatic practice" in the Iranian crisis, and Peking's diplomats privately have acted to buttress that position.