Sen. Sam Nunn (D-Ga.) said yesterday that President Reagan is inaccurate in saying that the Soviet Union has a thorough advantage over the United States in offensive nuclear weapons systems, and he urged Reagan to study the facts before he meets with Soviet leader Mikhail Gorbachev in November.

"The president needs to sit down with the Joint Chiefs and learn about our submarines, about our aircraft carriers, about our tactical air, about our cruise missiles, about our bombers and other advantages," Nunn said on ABC-TV's "This Week With David Brinkley."

Reagan administration officials have said that U.S. cruise missiles, submarine-based missiles and bombers are superior to Soviet models.

But last week, pressing the case for no compromise in research, development and testing of his proposed Strategic Defense Initiative (SDI), sometimes called "Star Wars," Reagan asserted in a nationally televised news conference that the Soviets held a complete edge in offensive nuclear weapons.

"At the moment," Reagan said, "I have to say that the United States -- in spite of some of the misinformation that has been spread around -- the United States is still well behind the Soviet Union in literally every kind of offensive weapon, both conventional and in the strategic weapons."

In part because of that advantage, Reagan said, he has so far ruled out a compromise suggested by Gorbachev in which the Soviets would make unspecified but sharp reductions in their offensive arsenal in return for limits on U.S. defensive weapons. Development of such weapons, Reagan has argued, would be the most effective way to curb an all-out arms race.

Nunn said the president should be better informed before facing Gorbachev.

"It's important for the president to know where our weaknesses are, but it's also very important as we lead to the summit for him to know where we're stronger," said Nunn, a moderate and a leading congressional specialist on defense issues as ranking minority member of the Senate Armed Services Committee.

Asked if he could see any advantage to such assertions by Reagan in advance of the scheduled Nov. 19-20 summit meeting in Geneva, Nunn said he could not. "Even if it were correct, which it is not," the senator said, "I think it would be a very poor way to begin negotiations."

Later in the broadcast, national security affairs adviser Robert C. McFarlane said that while the Soviet Union might not be ahead in every category, Reagan was not off the mark.

"In asserting that the Soviet Union has, where it counts, a substantial advantage, the president was absolutely right," McFarlane said. "That key measure of stability during crises of the nuclear balance favors the Soviet Union without any question."

McFarlane said, however, that despite Reagan's insistence on no compromise on development and testing of SDI, "there is a very good prospect that there will be some kind of arms agreement in the next year's time."

The two nations have mutual interests in strategic defense, fear of the consequences of nuclear actions by third nations and reducing offensive arms systems, he said.

McFarlane repeated Reagan's argument that the SDI should not become a "bargaining chip" in arms-control talks because it is the best potential deterrent to a continued buildup of nuclear weapons and because Soviet offensive strength is increasingly immeasurable.

Nunn said "vigorous research in SDI" should go on "unless the Soviets come up with radical reductions" in their offensive arsenal. "Then we still go forward with research . . . but we have to . . . be willing to negotiate where we're going in terms of deployment and testing and development."