Marshall D. Shulman, the State Department's senior expert on Soviet affairs, testified yesterday that the administration is prepared to seek congressional approval of trade benefits for China but not for the Soviet Union.

This represents a substantial reversal for the policy that Secretary of State Cyrus R. Vance had hoped to follow in this sensitive area. Vance, and his adviser Shulman, had intended to grant the trade benefits to both communist powers at about the same time to demonstrate an even-handed American policy toward both.

Testifying to the House Foreign Affairs Committee, Shulman said the administration would seek most-favored-nation tariff status only for China at this time because it would not be politically feasible to expect Congress to approve MFN for the Soviets.

In a long statement on the state of Soviet-American relations, Shulman told the committee that in this period of "extraordinary turbulance in international politics," the two super-powers have found it "particularly difficult to moderate the competitive aspects" of their relationship.

Shulman listed the many points of tension between the two countries, and noted too that the Soviets have made some gains with joint military interventions with the Cubans in third-world countries. However, he added, "It appears probable that these gains may prove as transitory as were earlier positions won and subsequently lost in the face of local nationalist resistance to the spread of Soviet control."

Under questioning, Shulman acknowledged that "there were some aspects" of the recent flap over Soviet troops in Cuba "that could have been handled better" by the Carter administration.