The CIA yesterday predicted that the Soviet Union will keep increasing its defense budget 5 percent a year, the same percentage Congress approved for the United States this year.

The agency, at a hearing before a House Intelligence subcommittee, did say it is possible that Soviet Military spending will slow down because of declining overall economic growth.

"The current and projected decline in Soviet economic growth raises questions about the U.S.S.R.'s ability to continue increasing defense spending," Robert Huffstutler, director of CIA strategic research, told the subcommittee.

While foreseeing little slowdown before 1985, Huffstutler added, "In the longer term, growing economic difficulties may push the Soviet leaders to reexamine their plans with a view to reducing the growth of defense spending."

Two possible economies would be to reduce the production rates of some weapons and agree to arms control agreements providing direct savings, the CIA specialist said.

He cautioned, however, that "we think it highly unlikely" that the Soviet leaders will reduce military spending to the point tthat it reverses "longstanding policy of continuing to improve their military capabilities."

He said Soviet defense spending, after allowing for inflation, has grown "an average of 4 to 5 percent a year since at least 1965." In contrast to this steady growth, U.S. defense spending has been up and down in that same period, with the Vietnam war pushing the totals up during the late 1960s.

Who has been responsible for the decline in U.S. military spending since Vietnam is a hot issue in the presidential campaign, with Democrats and Republicans blaming each other.

A related issue is whether the CIA has been providing accurate comparisons of U.S. and Soviet military spending, a topic Chairman Les Aspin (D-Wis.) said the subcommittee would question the agency about in closed session.

The CIA provoked a flap in 1976 by announcing that it was nearlly doubling its estimates of how big a slice of the Soviet gross national product was going for defense. Instead of the old 6 to 8 percent estimates, the CIA said, the new estimate was between 11 and 13 percent.

However, in 1978 the CIA attributed much of the increase to the fact that the Soviets were getting less bang for the buck because "Soviet defense industries are far less efficient than formerlly believed."

Yesterday the agency stuck with its 11 to 13 percent of GNP for the 1965 through 1978 period, but raised the estimate to between 12 and 14 percent for 1979 because the Soviet economy sagged that year.

The estimated annual growth rate, after allowing for inflation, in the Soviet defense budget was estimated at between 4 and 5 percent in rubles.

The CIA estimates how much it would cost the United States and the Soviet Union to duplicate each other's military establishment, with one comparison expressed in rubles and another in dollars.

The CIA's cumulative estimates in dollars from 1970 through 1979 were $1.135 trillion for the United States and $1.460 trillion for the Soviet Union, a difference of about 30 percent. In 1979 alone, the CIA estimated, the Soviet Union spent $165 billion on its military, about 50 percent more than U.S. expenditures.

Aspin said during a break in the hearing that CIA dollar comparisons are distorted because the agency puts Soviet soldiers on U.S. salaries.

Under that kind of figuring, Aspin said, "the Chinese would be spending us into oblivion" if their huge army was considered to be paid U.S. military salaries for making dollar comparisons.