The Soviet Union, with little fanfare, has considerably relaxed its policy on Jewish emigration this year, allowing more than 30,000 Soviet Jews-twice the number permitted in recent years-to leave the country.

Western diplomats and Jews here believe the sharp and unexpected upturn relates directly to two major Kremlin goals: ratification by the U.S., Senate next year of a new Soviet-American strategic arms agreement and easing or repeal of U.S. legislation linking Soviet-American trade to Jewish emigration.

Not since 1973, when almost 35,000 Jews were allowed to leave the Soviet Union, has the pace of Kremlin-sanctioned emigration reached such levels.

That was the last year before passage by the U.S. Congress of the Jackson-Vanik amendment, tying bilateral trade to Jewish emigration, brought an abrupt decline in the numbers of Jews allowed to leave this country. In the four years since enactment of the Jackson-Vanik amendment, Jewish emigration has averaged only 16,000 a year.

Officially, the Soviets have denied to recent U.S. Senate and business delegations visiting that there is any linkage between Soviet emigration policies and any other aspect of Kremlin relations with the outside world.

Yet, the emigration issue looms as a key aspect of any eventual Senate consideration of a new nuclear arms agreement and the Kremlin has indicated that it realizes ratification could hinge on a few key votes.

In addition, 12 U.S. senators and the secretaries of both treasury and commerce, who visited here this month made it clear that possible repeal of the Jackson-Vanik amendment can be considered only after Senate approval of a new SALT treaty.

In the opinion of Western diplomatic sources, the Soviet leadership, while conceding nothing to its critics, now recognizes privately that the restrictive emigration policy has major damage to the image of the Soviet Union both in the United States and in the rest of the West.

These sources suggest that the Kremlin now wants to improve the atmosphere with the United States, where the outcry against barring Jews from leaving has been the strongest.

U.S. disapproval represents the biggest impediment to Soviet interests in easing their massive defense burdens through a new strategic arms agreement and also through trade concessions that could lead to improved technology to modernize the faltering Soviet economy.

Western analysts believe that the Soviet Union, with an economy about half the size of the United States, spends nearly as much or more perhaps above $130 billion-on defense. The Soviet economy, while expanding yearly, has experienced sharply reduced growth rates this decade and faces reduced manpower supplies in the 1980s.

Some informed sources here suggest that the altered Kremlin policy toward the sensitive issue of Jewish emigration stems from the recent emergence-and acceptance-of China in the West as a country eagerly and amiably seeking both close diplomatic ties with Western capitalist nations and their technological know-how to modernize a long-isolated and backward economy. Peking and Washington are to establish normal diplomatic relations Monday after 29 years' estrangement.

Analysis of this year's Jewish emigration figures shows that until July, when the Chinese break-out from years of isolation from the West began in earnest, Jewish departures from the Soviet Union averaged about 1,900 per month. Since then, the monthly average has been about 3,200.

No one here is willing to link so closely official Soviet emigration policies with Kremlin worries over the international maneuvers of Peking, its bitterest and perhaps most feared adversary.

But among Jews here, who in record numbers are seeking invitations from Israel to emigrate to the Jewish state, there is the opinion that the Soviets are trying to readjust their national image in the face of possibly uncritical Western acceptance of a seemingly liberalized China whose own restrictive emigration policies are little known or understood in the West.

The 30,000th visa for this year was issued Wednesday in the small Dutch legation here as several hundred Soviet Jews, clutching their hard-won Soviet emigration documents, clustered in sub-zero cold outside.

Dutch officials, who have represented Israel here since the Soviets broke ties with the Jewish state during the 1967 Middle East war, refuse to discuss the numbers of emigrants or to speculate on why this year's total will be twice that of the last four years.

Soviet authorities now find themselves facing a remarkable new phenomenon: Requests by Soviet Jews to Israel and other countries for the necessary invitations to emigrate have averaged about 15,000 a month since October, far exceeding any previous monthly average since 1968, when exit interest among the Soviet Union's estimated 2.1 million to 2.7 million Jews first surged following the six-day Arab-Israeli war.

Jews and Western sources say the intense drive by Jews to leave stems from a variety of factors, including increased anti-semitism in official Soviet life, reports from friends and relatives living abroad that life in the West is not as bad as Soviet propaganda depicts it and in fact is a great deal better, and fears that Soviet visa authorities may suspend emigration applications altogether in 1979.

Soviet officials themselves explain the rise in invitation requests to what they see as false hopes of Middle East peace prompted by the Camp David accords.

Whatever the reasons for the attempted exodus, the only way the Soviets can avoid creating a large new category of "refusedniks" - Jews denied exit visas ostensibly for state security reasons - is to approve the emigration requests.

"The people leaving are all first-time applicants," one well-informed Western source asserted."There are virtually no real 'refusedniks' being added."

Jews and Western sources believe that the present small group of refusedniks, perhaps no more than a few thousand across the Soviet Union, has become an acute problem for Soviet authorities, who are thought to have denied them permission to leave as a warning to other Jews.

But instead, Jewish emigration interest is rising, and the cause of the refused Jews has been taken up with increasingly telling effect by Westerners.

Many refusedniks are scientists and in recent years a number of Western scientific groups have spoken out against the discrimination toward their Soviet colleagues or denounced the Kremlin. In a few cases, Westerners have barred Soviets from participating in symposia or made Soviets reluctant to come because they feared a confrontation.