The Soviet Union is preparing its economic plan for the rest of this decade, and there are clear signs that virtually all Soviet contracts for major development projects are going to be awarded to West European and Japanese companies, to the virtual exclusion of U.S. industry.

While Moscow long has been used to dealing with the Western Europeans and Japanese on major commercial contracts, with most U.S. trade being in grain and other commodities, Kremlin planners in the past appeared to have been at least open to discussing projects with the Americans.

But Soviet sources indicate that the Kremlin now is leaning toward and perhaps already has made a decision not to include American companies in long-term development contracts.

The stakes are substantial. As the Soviet economy has begun to show modest growth after the slump of the early 1980s, several projects worth $1 billion or more will be undertaken, for the first time, by foreign firms from design to start-up under so-called "turnkey contracts." Virtually no American firms are involved in current commercial negotiations for such multimillion-dollar projects.

Well-informed Soviet trade officials say this pattern has prompted an internal debate here, with important elements in the Foreign and Foreign Trade ministries arguing for American participation, both for economic and political reasons.

But the prevailing view among high-ranking officials in the Central Committee, State Planning Commission and Foreign Trade Ministry is that commercial relations with the United States should be limited to specific, short-term contracts.

According to this view, the myriad political differences between the two superpowers rule out any guarantee that politics will not be injected once again into East-West trade. Consequently, it is argued, the Soviet Bloc nations should develop their economic plans in a way to reduce their vulnerability to any possible future U.S. boycott.

In Washington, senior Reagan administration trade officials expressed surprise that the United States has been left out of the Soviets' new development plans, although they acknowledged that Moscow was questioning America's reliability as a trading partner. But the U.S. officials said they thought those doubts had been overcome during a January visit to Moscow by Commerce Undersecretary Lionel H. Olmer, who reopened high-level trade talks betwen the two countries and reportedly was assured that no decisions had been made on foreign participation in the new five-year plan.

Any decision by the Soviets to exclude U.S. companies from their new development plans would hurt the prospects of Commerce Secretary Malcolm Baldrige going to Moscow later this year. The trip has been agreed upon as part of the Reagan administration's current strategy to build new bridges to Moscow, but no date has been set for it, Washington Post staff writer Stuart Auerbach reported.

The Kremlin is not seeking economic self-sufficiency. This was explicitly rejected at last year's summit of Comecon, the Soviet Bloc economic alliance.

On the one hand, Moscow has demanded that its allies provide greater investment of technology, capital and even labor to assist in the extraction of energy resources from Siberia. This has led to a greater integration of the Soviet Bloc economies and an increase in their trade.

On the other hand, Moscow is now seeking to deal with Western Europe and Japan. Some of America's key allies have been reluctant to impose politically motivated sanctions against the Soviet Bloc. Some of these U.S. allies also depend heavily on their exports to the East.

For example, of West Germany's total exports, 40 percent of mining equipment, 10 percent of valves, 20 percent of shoe and leather equipment, 25 percent of metal-cutting machinery and 10 percent of textile machinery go to the Soviet Bloc.

In the 1970s, there were expectations here that American companies would be involved in major development projects. Only a few of the planned projects have materialized, such as the Kama River truck factory, while several others already agreed upon collapsed after the Soviet invasion of Afghanistan.

Besides former president Jimmy Carter's trade restrictions over Afghanistan, the Soviets also were hurt by President Reagan's hard-line economic policy during his first term, including his attempt to delay the construction of the pipeline bringing Siberian natural gas to Western Europe.

The Soviet Union's overall foreign trade showed a $9.9 billion surplus in 1984. Although part of this surplus involves Soviet exports to Third World countries and is not readily converted to cash, it nevertheless puts Moscow in a more comfortable financial situation.

Under the five-year plan that is now being drafted for 1986-90, the Soviets have some very big contracts to dangle before western businessmen. Politburo member Mikhail Gorbachev did that during his visit to Britain in December, and other senior officials have done so on numerous occasions since.

A list of major projects includes:

* A $1 billion metallurgical complex at Volzhsk, on the Volga River, to produce, among other things, large-diameter pipes. Bidders for this project are the West German firm of Mannesmann and two Italian firms, Italimpianti and Finsider.

* A chemical and plastic complex whose total cost will exceed $1.2 billion. This is a "turnkey contract." Moscow is now negotiating with the British firms of John Brown, ICI and Davy McKee.

* A plant for production of chemical equipment whose total construction cost is estimated at $1 billion. The current talks involve the Italian firm of Montedison.

* A billion-dollar project to build and equip a metallurgical plant at Orel, west of Moscow. The main bidders for this "turnkey project" are the Austrian firm of Voest Alpine and the Italian firm of Danieli.

* A multimillion-dollar plastics plant involving an Italian firm, Snia.

* A multibillion-dollar project for gas and oil extraction on the Far Eastern island of Sakhalin. According to Soviet officials, this project would require imports of equipment exceeding $2 billion. Several Japanese firms are said to have been approached.

* A gas and oil development project in Kazakhstan whose total cost exceeds $1 billion. Officials said engineering fees for the Karachagansk project exceed $100 million. Mannesmann, the French firm of Teknip and unnamed Canadian firms are reported to be vying for this project, as well as a similar project at Tengiz, also in Kazakhstan.

* A new gas pipeline, the so-called Ymaburgska, to run to the border of Czechoslovakia. The multibillion-dollar project will mainly involve Soviet and East European firms. Like the Siberian pipeline, it will involve massive imports of large-diameter pipes and compressor stations from Western Europe and heavy construction machinery from Japan.

* Finally, an ambitious and still nebulous project to develop the continental shelf on the Barents Sea. The project's cost is estimated by Soviet officials to be greater than any of those mentioned earlier. Soviet officials said Finnish and Swedish companies are "bombarding" Moscow with proposed offers.

The 1986-90 five-year plan also involves a series of projects whose cost runs under $1 billion. Nevertheless, officials here say, these smaller projects involve substantial amounts of money.

Among these are plans for construction of several wood treatment plants and paper mills along the new Baikal-Amur railway, modernization and reconstruction of automobile plants at Gorki, Moscow, Togliatti and Zaparozhe, and many food processing facilities. Talks about these projects involve firms from West Germany, France, Britain, Japan, Austria, Finland and other West European countries.

Soviet officials said the U.S. trade delegation led by Olmer achieved no major progress. According to Soviet figures, U.S.-Soviet trade last year was still below the 1979 level, with the United States falling from second to seventh place on the list of Moscow's capitalist trading partners.