A Senate hearing on U.S. aid to Syria was postponed abruptly yesterday after the Reagan administration expressed concern that the session could complicate Secretary of State George P. Shultz' peace-making effort in the Middle East.

Sen. Alfonse M. D'Amato (R-N.Y.), who had pushed for the hearing, called the turn of events "totally unacceptable."

The Senate Appropriations subcommittee on foreign operations, of which D'Amato is a member, had planned to question M. Peter McPherson, administrator of the Agency for International Development, about further dispersal of aid to Syria. D'Amato had made clear his opposition in a brief exchange on the subject with McPherson March 3.

Syria's policy is likely to be a crucial factor in the success or failure of Shultz' drive to obtain withdrawal of foreign forces from Lebanon, according to reports from the Middle East. While Shultz seems to be making progress toward agreement between Israel and Lebanon, increasingly negative statements from Damascus cast growing doubt that Syria will accept this accord as a basis for withdrawing its forces.

Shultz is expected to meet Syrian President Hafez Assad as soon as the Israeli-Lebanese arrangement is well enough advanced. Some reports said Shultz may fly to Damascus at the end of this week.

U.S. relations with Syria improved after a disengagement agreement between Israel and Syria was negotiated in May, 1974, by Henry A. Kissinger, then secretary of state, and a U.S. aid program for Syria was started. However, relations deteriorated in the late 1970s as Syria opposed the Camp David accords and other U.S. diplomacy involving Israel and Egypt.

The last new U.S. aid commitment for Syria was in fiscal 1979. In April, 1981, following anti-American speeches by Assad, the Reagan administration--with congressional prodding--stopped aid that had not already been placed under contract.

This action left $146 million in the aid "pipeline" already committed, often through U.S. companies, but not yet spent.

Undersecretary of State Lawrence S. Eagleburger said yesterday that he had expressed concern to a U.S. senator, reported on Capitol Hill to have been D'Amato, about having Syrian aid discussed while Shultz is in the Middle East. Eagleburger declined to elaborate on the basis for his concern.

McPherson, who had been scheduled to testify yesterday morning, sent word Monday evening that he could not appear and offered a lower-level substitute. McPherson said later that a "misunderstanding" was involved and that he will testify at a mutually convenient later date.

D'Amato, in a statement, said he is determined to pursue the Syrian aid issue. Subcommittee Chairman Robert W. Kasten Jr. (R-Wis.) said a new date for McPherson's testimony will be arranged.