The Reagan administration will try to expand trade with the Soviet Union, which has been cut in half since 1979 as a result of U.S. sanctions, as part of an overall plan to ease tensions with Moscow, Commerce Undersecretary Lionel H. Olmer said yesterday.

Olmer will meet Jan. 8 in Moscow with Soviet Deputy Foreign Trade Minister Aleksei Manzhulo in the highest-level trade encounter between the United States and the Soviet Union since 1978.

U.S.-Soviet trade took a nosedive the next year as Moscow's invasion of Afghanistan brought sanctions that were continued and in some cases expanded after the imposition of martial law in Poland.

U.S. exports to the Soviets totaled $2.3 billion last year, mostly in farm products, compared with $4.5 billion in 1979. But Secretary of State George P. Shultz, who has been pressing for increased commercial contacts between American businessmen and the Soviets, indicated in recent speeches that he opposes linking trade sanctions to actions by Moscow that the United States opposes, such as the Afghan invasion and martial law in Poland.

The decision to try to improve trade with the Soviets was reported yesterday in The New York Times.

"We don't look to a massive surge in U.S. exports by any means," said Olmer. "The potential isn't great, but it's significant to some companies."

American companies doing business with Moscow estimated U.S. export losses from government restrictions at about $10 billion a year.

But there are potential storm clouds to any administration efforts to improve trade relations with the Soviet Union.

One is an International Trade Commission study due Monday that could find the Soviets use slave labor to produce products for exports, which would spark another trade crisis.

The other, administration sources said, is the possibility the Soviets are supplying Nicaragua's Sandinista government with new MiG-21 combat planes, which the California White House has denounced as an escalation of Moscow's interference in Latin America.

While the Commerce and State departments are pushing increased trade with Moscow, moreover, the Pentagon is wary that it could lead to the diversion of sensitive technology that can be used by the Soviet military.

Even the sale of simple technology has been opposed by some elements of the administration and more conservative congressional leaders.

They point to trucks made by an American-supplied plant that reportedly were used to carry troops for the Afghan invasion despite a clause in the contract against their military use.

Olmer emphasized that sensitive technology -- including oil and gas drilling equipment and technology the Soviets want to explore Barents Sea fields -- is not part of the shopping list the United States is prepared to offer Moscow.

But at the urging of U.S. Ambassador to Moscow Arthur A. Hartman, Olmer has prepared a 13-page booklet telling American businesses what they can and cannot sell to the Soviets.

These include goods that generally can be sold without licenses -- which include some basic high technology products, farm machinery, chemicals and machine tools -- as well as higher technology goods that need licenses and those that are unlikely to win approval.

The booklet was prepared in May, at the same time the president quietly approved the extension for another 10 years of the Industrial and Technical Cooperation Agreement with the Soviets. The agreement, originally signed in 1974, expired this year. Olmer's Moscow meeting will take place as part of the Joint Commercial Commission authorized by that agreement, but suspended by the United States in 1979 after the Soviet invasion of Afghanistan.