The Soviet Union's $100 billion air defense network would be "totally useless" against U.S. cruise missiles, and for Moscow to spend another $50 billion on it would be throwing good money after bad, the Pentagon's research director said yesterday.

William J. Perry, in drawing that conclusion after 10 months of testing against simulated Soviet defenses, said the U.S. cruise missile offese can easily stay ahead of the Soviet defense for the foreseeable future.

However, he predicted the Soviets will not give up. "They will blunder ahead and build this [new] strategic air defense system" as part of moving ahead "on all fronts" militarily, he said.

In reflecting on his prediction, Perry said he would welcome the Soviets wasting a lot of money of cruise missile defense because this would leave less money for new offensive weapons.

Perry's upbeat report comes at a time when President Carter, who last year chose the cruise missile over the Air Force B1 bomber, is trying to convince skeptics that signing a new arms control agreement with the Soviet Union would be an acceptable risk.

Asked if there was any linkage between yesterday's Pentagon press conference and the administration effort to sell a new arms control agreement, Perry replied "No."

Both he and Thomas B. Ross, Pentagon spokesman, said Perry's presentation wa sin response to reporters' repeated requests for an undate on the cruise missile tests.

The Pentagon research director went through each of the present rings of Soviet air defense and said tests showed each would fail to stop an attack by the thousands of cruise missiles the United States intends to build.

The Soviets, Perry said, would have to: replace their 10,000 ground radars with ones installed on aircarft, build a new fleet of some 1,000 interceptor planes with look-down radar and new missiles, and deploy a better-ground-based antiaircraft missile.

After spending $50 billion doing all that to blunt an attack by today's cruise missile, the research director continued, the Soviets would be confronted with an improved U.S. cruise missile. It would be more difficult to track and stop, maneuverable enough to dodge new defenses, and equipped with penetration devices.

In flight tests against simulated Soviet defenses, Pentagon specialists discovered that Soviet longrange radars could not detect a cruise missile while it was far away and were "very poor" at pinpointing the location of the intruder, perry said.

Soviet radar for close-in acquistion and guidance of fighter planes also "generally failed" to find the cruise missle flying some 200 feet above the ground, Perry said. If these radars did detect the missile, it was generally too late to do any good.

Without losts of radar help, the research director continued, present-day Soviet fighter planes armed with heat-seeking and radar-directed missiles would have trouble stopping incoming cruise missiles.

Soviet antiaircraft missiles on the ground also turned out to have "small lethality" against the cruise missile, Perry said. Clustering the new Soviet SA10 missile around a missile base or other high-priority target could provide significant defense, however.

The United States plans to build about 3,000 cruise missiles costing $1 million each, and put them aboard B52 bombers at first and perhaps inside jumbo jets, as well as in ships, submarines and on the ground. Perry said the whole cruise missile program - planes as well as missiles - would cost about $10 billion. The first cruise missiles are scheduled to be deployed in 1982.

"I would be very nervious about cruise missiles if I were a Soviet defense planner," Perry said.