President Reagan's former arms control chief, Eugene V. Rostow, yesterday criticized the administration's public disclosure of possible new arms control proposals as "a terrible way to negotiate with the Soviet Union."

Appearing before the Senate Foreign Relations Committee with three other former directors of the Arms Control and Disarmament Agency (ACDA), Rostow made this charge when questioned about a recent letter to the committee from the agency's current director, Kenneth L. Adelman.

In the unclassified letter made public by the committee, Adelman suggested that the United States would forgo deployment of 100 new MX missiles only if the Soviets give up all 650 of their existing SS18 and SS19 missiles.

Some committee members said they think the letter may signal, for the first time, a willingness to bargain over the MX. But others said it is so outrageous that the Soviets would not take it seriously and that such proposals contribute to a view that the administration is not serious about arms control.

Administration officials said yesterday that Adelman's answer came in response to a hypothetical question from the committee and was not intended as a specific proposal to Moscow.

Under questioning by Sen. Joseph R. Biden Jr. (D-Del.), Rostow agreed that even such hypothetical proposals "hurt" U.S. negotiators at the Geneva nuclear arms control negotiations between the United States and the Soviet Union.

He explained later that he was not making a judgment on the position itself, but saying that such disclosures can unnecessarily worsen the atmosphere of the negotiations if made publicly here before the Soviets are informed and before they are completely considered by the administration.

Rostow decried what he called a tendency to "bargain with ourselves" and "back-seat driving" by Congress in the formulation of U.S. negotiating positions.

But Biden shot back that it was precisely because so many people did not believe Reagan was serious about arms control that Congress and the public were so involved.

Former arms control agency directors Paul C. Warnke from the Carter administration and Gerard C. Smith from the Nixon administration were also critical of Reagan's newly modified proposal at the Strategic Arms Reduction Talks (START).

Warnke said Reagan seems to believe "we can make the Soviet Union cry uncle at the bargaining table" by deploying 100 MX missiles. "This is not bargaining. It is, instead, the arms race theory of arms control, and it won't work," he said.

Smith called the Reagan plan essentially un-negotiable. "Have we not learned by past experience that one-sided proposals cannot work and will only kill time, which is a wasting asset?" he asked.

Both said the failure to include cruise missiles was a major flaw in the U.S. position.

Rostow, although fired by Reagan in January, was generally supportive of administration policy. He warned that Reagan, as an election year approaches, "is coming under increasing political pressure to settle for a poor agreement," that "public opinion can be easily whipped up" and that "each of us therefore should be careful about what we are doing."

Rostow said that while there is always intense debate here about the American stance, a major reason for lack of progress is that Soviet positions are not serious and "we have a profound psychological and political resistance to accepting the reality of that."