President Reagan yesterday named Ronald F. Lehman, his deputy assistant for national security affairs, to replace former senator John G. Tower as chief negotiator for strategic arms reduction with the Soviet Union.

Lehman, 39, who already has the rank of ambassador, was deputy arms negotiator under Tower last year and, at the same time, was a member of the National Security Council staff handling nuclear weapons matters. In a recent NSC staff reorganization, Lehman was made deputy to national security adviser John M. Poindexter in charge of defense and arms matters.

The State Department said no decision has been made on whether Lehman will give up his NSC post to devote full time to the Geneva negotiations. The fifth round is scheduled to start May 8. Special arms adviser Paul H. Nitze said Thursday the Soviet negotiating position on strategic arms appears to be "frozen" and confined to "abstractions and generalities."

"Lehman provides continuity and an expertise needed by this administration," a colleague said.

Secretary of State George P. Shultz described Lehman yesterday as "a person of tremendous ability, of great knowledge and background in this field and with genuine experience on this task," and said Tower had recommended him.

Before moving to the White House, Lehman was deputy assistant secretary of defense for nuclear forces. Like many other administration officials in the national security field, he was on the staff of the Senate Armed Services Committee when Tower, a Texas Republican, was its chairman.

Tower, whose resignation as arms negotiator was announced this week, said Thursday he was "somewhat disappointed" by lack of progress in the most recent round of talks but has not lost hope.