A House-Senate conference committee considering the $302.5 billion military spending bill has voted to adopt a provision permitting imposition of the death penalty under military law for espionage committed during peacetime, and to double the Pentagon's authority to conduct lie detector tests.

Adopting an amendment that was passed overwhelmingly by the House, the conferees considering the fiscal year 1986 Defense Department authorization bill approved a measure to permit execution of military personnel convicted of espionage during peacetime. Under current law, the maximum penalty is 10 years' imprisonment.

However, the conferees rejected a Senate-passed measure that would have made mandatory a sentence of either life in prison or execution for those convicted under civilian law of spying for the Soviet Union or one of its allies.

Instead, the conferees, who reached their final agreement Thursday night, went along with a House measure directing Secretary of Defense Caspar W. Weinberger to study and report to Congress on the advisibility of restoring capital punishment in espionage cases. Weinberger has said he strongly supports such a move.

On the issue of lie detectors, the conferees voted to continue next year a year-old pilot program under which the Pentagon is authorized to conduct 3,500 "counterintelligence" polygraphs to ferret out spies and potential spies in the military and among employes of defense contractors.

The lie detector measure approved by the conference committee would expand the scope of the existing polygraph program to include those authorized to see "top secret" information as well as those with clearance for "special access" programs, the highest category of classified information.

In the subsequent fiscal year, the program would be doubled to permit the Pentagon to conduct 7,000 such lie detector tests.

The bill now returns to the full House and Senate for final approval.

According to the most recent Pentagon figures, 98,700 Defense department employes and another 9,576 contractor employes hold "special access" clearances, and 454,851 Defense Department employes and 128,405 contractor employes are cleared to see "top secret" information.

L. Britt Snider, the Defense Department's director for counterintelligence and security policy, said the Pentagon expects to conduct 600 of the 3,500 polygraphs authorized for this fiscal year. But Snider said that the Pentagon might be able to conduct all of the 7,000 tests permitted in 1987 if the agency is able to meet its goal of training 40 additional examiners.

Action on the death penalty and polygraph measures was shaped by the recent arrests of John Anthony Walker Jr. and three other Navy men on charges of funneling national defense secrets to the Soviet Union. The arrests and ensuing reports about the extent of possible damage to national security spurred Congress to take steps to combat espionage.

The final version of the bill also authorized $595,000 next fiscal year for research into polygraphs.

Under an amendment that won overwhelming approval in the House, the Pentagon would have been given authority to subject to polygraphs any of the 4.3 million military and civilian employes cleared to see classified information. Lie detector tests would have been mandatory for those seeking the high-level "special access" clearance, and congressional approval to conduct such tests, which has been a source of struggle in the past between legislators and the Pentagon, would have been continuing rather than limited to the next two years.

The conference committee compromised between the House measure and the existing pilot program by expanding the use of lie detectors to include the "top secret" category and by increasing the number of tests that could be conducted in 1987, but restricting authorization to two years.

"I think it's a pretty good compromise," Sen. Sam Nunn (D-Ga.), the ranking minority member of the Senate Armed Services Committee and one of the chief architects of the final agreement, said in an interview yesterday.

If the Pentagon were permitted to subject to lie detector tests all employes with clearances, "I think it would have been unworkable," Nunn said. He said that if such a large number of people were possible subjects, the threat of being required to take a polygraph "would have been so remote" that the program's "deterrent effect" would have been minimal.

But Rep. C.W. Bill Young (R-Fla.), who sponsored the House amendment, said he was "disappointed" by the final outcome. He noted that under the compromise, Navy Seaman Michael Lance Walker, one of the men charged with espionage, would not have been subject to a polygraph because he held only a "secret" clearance.