President Reagan's national security affairs adviser, Robert C. McFarlane, said yesterday that there had been a constructive "evolution" in the Soviet approach to nuclear arms control during the second round of talks that concluded at Geneva on Tuesday.

McFarlane told reporters it would be "unrealistic" to expect "dramatic change" while Soviet leader Mikhail Gorbachev is preoccupied with internal economic issues. But he said the Soviets had been willing to present "a broad concept" for reducing strategic nuclear forces, which they had been unwilling to do in the first round, even though they did not respond to specific U.S. proposals for reductions.

"We have seen a few signs that might ultimately prove to be promising," McFarlane said. "For example, in this round the Soviets were marginally less polemical than in the first round, and in some areas they have begun to respond to our effort to engage them in a serious dialogue."

McFarlane's mildly optimistic assessment came after he conferred Thursday with the U.S. arms control negotiating team headed by Max M. Kampelman and met for 20 minutes yesterday with Reagan at Bethesda Naval Hospital. McFarlane expressed hope that the Soviets will be "ready for a real give-and-take process" when the Geneva talks resume in mid-September.

The McFarlane evaluation differed with the pessimistic Soviet evaluation of the talks earlier in the week. Tass complained that the United States had displayed a "patently negative nature" in the second round and Viktor Karpov, the chief Soviet delegate in Geneva, repeated a warning from Gorbachev that the Soviets would reassess participation in the negotiations unless there was progress.

Despite his effort to be positive, McFarlane offered no hope of any U.S. concession on the issue which has proved the sticking point of the talks. The Soviets want Reagan to scrap the proposal for missile defense that he calls the Strategic Defense Initiative (SDI) and is more commonly known as "Star Wars." The Soviets contend that testing elements of a missile defense would violate the 1972 Antiballistic Missile (ABM) treaty.

McFarlane called the Soviet position "unreasonable" and "wrongheaded" and said the Soviets were proceeding with Star Wars research while asking the United States to abandon it.

"While they deny our right to do what the ABM treaty clearly allows, they assert their right to violate the same treaty," McFarlane said. "In short, it's a charade; it's a propaganda ploy."

But McFarlane declined to use the word "propaganda" to describe a Soviet suggestion that there could be serious reductions in offensive weapons if the Reagan administration would agree to limit the SDI to research. Instead, he called upon the Soviets to "put your money where your mouth is -- put it on the table" at Geneva.

At a White House briefing, McFarlane also objected to the "inference" of a New York Times article that said a revised assessment by the Central Intelligence Agency has determined that the Soviet SS19 missile is too inaccurate to pose a threat to U.S. missile silos. This assessment, which differs from some Pentagon estimates, was first disclosed by the weekly magazine National Journal.

"The inference of that Times article is misleading in the sense that the clear imbalance . . . which favors the Soviet Union exists without regard, really, to the SS19's precise capabilities," McFarlane said. "The article, by omission, ignores the SS18 which, comprising some 3,000 ballistic missile warheads as it does, establishes a very, very solid capability against our systems."