Proposals to use lie detector tests to search for spies and to impose the death penalty for espionage would violate fundamental constitutional liberties, a top official of the American Civil Liberties Union said yesterday.

Morton Halperin, Washington director of the ACLU, called on Congress to reject such proposals as unconstitutional.

The proposals were contained in amendments to defense authorization bills. They were sparked by concern over the Walker spy case, in which four persons are accused of selling Navy secrets to the Soviet Union.

"Congress, when it returns after the recess, should strip these measures from the bill and start attacking the genuine problems such as overclassification of information and excessive numbers of employes with access to sensitive information," Halperin said in a written statement.

Halperin said he opposed the death penalty on constitutional grounds and believed it would not deter spying. He said major spies would be traded or would get their sentences reduced by telling the government what secrets were passed.

"In the end, the death penalty could be used against those who have caused little damage to national security," he said.

Random use of polygraphs to detect spies violates "the presumption of innocence, due process and the privilege against self-incrimination," Halperin said. Real spies, he contended, can learn to fool a lie detector test.

Before the Fourth of July recess, the House approved a proposal making espionage by military personnel punishable by death. It also voted to give the Defense Department wide authority to administer polygraph tests. House and Senate versions of the defense spending bills are before a conference committee.