The helping hand of the Reagan administration, aimed at sparing Mikhail Gorbachev more anguish from the Soviet occupation of Afghanistan, has angered congressional hard-liners and a few high officials who fear the United States might even end up a net loser after a Soviet exit on Soviet terms.

What agitates these officials most are continuing suggestions from Secretary of State George Shultz that U.S. aid to the anti-Soviet mujaheddin will stop long before final withdrawal of the 115,000-man Soviet occupation force. ''As withdrawal proceeds . . . you don't have the need for that continued support and it would cease,'' Shultz said last Friday.

In fact, however, neither Shultz nor anyone can prove anything about the ''need for support'' or whatever else may happen if and when Soviet troops withdraw. Instead of rosy scenarios, his critics insist that the United States make no commitments on aid until the Soviets actually pull out, instead of just talking about it. Ending U.S. aid would remove the one prod that has driven Gorbachev to decide his country can no longer hack it in Afghanistan.

The original U.S. commitment to cut its aid when Soviet withdrawal begins was made two years ago when the mujaheddin were far more vulnerable than today. But there is nothing automatic about the cutoff. It is conditioned on U.S. acceptance of a ''satisfactory'' agreement for a fair and just political settlement guaranteeing self-determination -- thus ending forcible Communist control. That gives the United States leverage that should be used.

The unhappiness of President Reagan himself with the Soviet pullout agreement made two years ago by administration diplomats is manifest. Just before his summit with Gorbachev, Reagan told the four television networks in an interview that he would do nothing to ''facilitate'' Soviet withdrawal. Asked whether he would agree ''not to supply'' the mujaheddin ''if the Soviets committed to get out of Afghanistan,'' Reagan's reply was unambiguous: ''I don't think we could do anything of that kind.''

''The elegant dandies of diplomacy seem to have put the old cowboy in his place once again,'' said Sen. Gordon Humphrey, a Republican gadfly on Afghanistan whose unremitting counsel has sometimes been highly effective even within the State Department. Humphrey told us that he found it hard to stomach the contradiction between Reagan's words and what the United States appears actually committed to do.

The prospect of a U.S. aid cutoff before the installation of a new, non-Communist government in Kabul is redolent of what happened to the United States in the Vietnam nightmare. That is a Vietnam-Afghanistan analogy quite different from the usual mirror-image comparison of the Soviet predicament in Afghanistan and the U.S. failure in Vietnam.

The United States accepted apparently ironclad guarantees from Soviet-backed North Vietnam in agreeing to the Paris Accords. The Accords were supposed to end the Vietnam War while protecting Saigon's independence from North Vietnam. Within months Hanoi, backed by the Soviets, broke every one of the Paris Accords.

As farfetched as that analogy may seem, administration critics of a premature U.S. agreement to cut off aid to the Afghanistan freedom fighters see deadly parallels: the geographic proximity of ready Communist power in both Vietnam and Afghanistan; the great distance of the United States from the scene; the political difficulty for the United States to start up a major military aid operation once it has been stopped, vividly demonstrated with the contras in Nicaragua; and perhaps most important, the lack of proof that Soviet promises in Afghanistan are more credible than Hanoi's promises at Paris.

But Reagan's advisers seem befuddled. At the senior White House staff meeting last Friday, John Negroponte, the deputy national security aide, claimed that if Soviet forces withdrew from Afghanistan within six months, Kabul's puppet Communist regime could not possibly survive. But if the Soviet armies remained for another year or 18 months, he said, the Communists might be able to hang on to power after Soviet troops leave.

Negroponte was a major but reluctant U.S. player in the Paris Accords ending the Vietnam War. He was conspicuously absent at their signing. If his healthily skeptical view now about Afghanistan is correct, other administration officials tell us privately, why in the world is Shultz even talking about the United States having to end all aid to the mujaheddin? The far better U.S. course, these officials say, would be an immediate escalation of military aid. That should add new pressures on the Soviets and give Gorbachev new reasons for a quick withdrawal.