The Reagan administration, in its first major research report on acid rain, yesterday called man-made air pollution the prime cause of the problem, but said there is still not enough evidence to prove that cuts in power-plant emissions will halt damage to lakes and streams in the Northeast and Canada.

The report contributed to anticipation about whether the administration will change its acid rain policy, which has caused diplomatic friction with Canada and political uproar in the Northeast. President Reagan recently asked Environmental Protection Agency Administrator William D. Ruckelshaus to reassess the policy as his No. 1 priority.

The administration has so far opposed legislation to combat acid rain, arguing that more research is needed before costly regulations are imposed on coal-burning power plants. While yesterday's report continued that cautious tone, its authors said uncertainty alone is no reason for policy makers to delay action.

"Science will never give you a conclusive, definitive answer on most environmental questions," said Assistant EPA Administrator Courtney Riordan, a participant in the 12-agency study. "It'll take you a few steps, but somebody has to make a great leap." The same was true in 1970 when Congress passed the Clean Air Act, he said, and there is "no evidence we acted too soon."

The 55-page report, part of a 10-year government study of acid rain, concludes that the problem threatens large portions of the South and West, not just the Northeast and Midwest as once thought.

Acid rain results when pollution mixes with moisture in the atmosphere, coming down in rainfall hundreds of miles away as sulfuric and nitric acid. Scientific studies have linked it to extensive fish kills, and there is evidence that it threatens crops, forest and human health.

But the report questioned many of these findings. It said only a "small number of lakes" had been destroyed, although in New York's Adirondack Mountains alone, studies show that all fish have died in 180 lakes. It also said evidence of crop and forest damage is shaky, despite studies indicating soybean losses in the Midwest and forest damage in the Northeast and West Germany.

And it said there is not enough data to show how regulating emissions in the Ohio Valley, home of major coal-fired power plants, will curb acid rain elsewhere.

Canadian officials, environmental groups and coal industry spokesmen all said the report shows little change from the administration's past position.

"It's a step in the right direction but it's like the Neanderthal coming out of the cave" for the administration to acknowledge a link between man-made pollution and acid rain, said Liz Barratt-Brown of the Natural Resources Defense Council.

"That has been the scientific consensus for years," she added, citing reports by the National Academy of Sciences, the 1982 Stockholm Conference of 21 nations, and many government and university scientists.