The possiblity that Western Europe had the Soviet Union will move ahead with the largest East-West trade, energy and construction project ever mounted has become a source of concern in the Reagan administration.

The concern is that construction of the 3,000-mile natural gas pipeline from the Yamal Peninsula in Siberia to Western Europe -- four times as long as the oil pipeline in Alaska -- will leave U.S. allies in Europe vulnerable to Soviet political pressure and thus place a potential strain on the Atlantic alliance during periods of tension with Moscow.

State Department officials acknowledged this week that Secretary of State Alexander M. Haig Jr. had called attention to the project during meetings here with West German Foreign Minister Hans-Dietrich Genscher. The official described Haig's coments as directed toward "the known U.S. concerns, which were held by the previous administration as well." Haig, he said, told Genscher the project must be looked at carefully in light of possible European vulnerability to additional Soviet pressure.

That vulnerability could come, first, through threats to the lucrative construction contracts with West European firms likely to do most of the work on the $10 billion project, then over the eventual supply of sizeable quantities of Soviet natural gas to West Germany, France, Holland, Italy, Belgium, Austria, Sweden and Switzerland.

The project has not yet received the green light, with the biggest remaining problem said to the Soviet willingness to agree to financial terms demanded by West European banks. But if it does move ahead, it will dwarf any previous economic and energy link between East and West.

Genschr, according to U.S. officials, gave assurances that the political vulnerability issue could be protected against and should not be a problem, and that there will be more time for allied consultations before the deal is completed.

French officials who visited Washington recently also said that while the political questions raised by the pipeline were serious, they could be properly managed. The Soviets do not have the technical know-how or the manufacturing capability to build the line themselves. And, though West European economies are being squeezed and the big construction and pipeline companies need the business, the French officials believe the focus of concern should not be on gaining political influence because of such contracts.

Rather, the French believe the key point to focus on is the degree to which European countries become dependent on the Soviet gas. These officials say they believe the level of Soviet gas in each country must be kept below a critical level that might make that country vulnerable to a Soviet threat to shut it off in a crisis. The French view is that this should be possible but that alliance consultations are necessary to make sure there is complete understanding.

"Having no gas is no answer" to Europe's energy problem, a French official said, so the point is to manage the project rather than cancel it.

West Germany would be the key financer and user in the project and it is Bonn's already widespread and complex web of trade and ethnic ties to Eastern Europe that has caused concern in the Reagan and Carter administrations. The question is whether West Germany is becoming too susceptible to pressure from Moscow.