The Marxist government of Angola is expected to release a captured American pilot in the next few days as a gesture in its efforts to establish relations with Washington, according to Jesse L. Jackson and congressional sources.

The pilot, Joseph Longo of Greensburg, Pa., was shot down April 21 while flying over southern Angola in a light aircraft. He was suspected of spying for South Africa, the official Angolan news agency said at the time.

The State Department said April 24 that it had been informed by the U.S. Embassy in Pretoria that Longo was ferrying a Beechcraft Bonanza owned and operated by a U.S. company, Specialized Service Inc., from the United States to Johannesburg, South Africa. The Bonanza reportedly was to be delivered to the National Airway Corp. in South Africa.

The department denied that Longo had ever been a U.S. government employe.

Jackson, a Democratic presidential candidate, and several members of Congress, led by Rep. Howard E. Wolpe (D-Mich.), urged a high-level Angolan delegation visiting here last week to release Longo as a goodwill gesture.

Jackson said he believes that Longo's release is "imminent."

At a press luncheon here June 11, the leader of the Angolan delegation, Afonso Van Dunem, said that Longo was being interrogated to determine why he had flown over a military zone. He said he was "quite sure" Longo would be released "if he tells the truth," and he indicated Angola had found no evidence that Longo was spying for the Central Intelligence Agency.

The United States has had no diplomatic relations with Angola since its independence from Portugal in 1974. The Reagan administration has insisted that it must first send home the estimated 37,000 Cuban troops stationed there.

The two governments have resumed talks over this and other issues, and a U.S. delegation is scheduled to go to Luanda, the Angolan capital, for a new round at the end of this month.

Jackson urged the administration to take Longo's release as "a signal for cooperation" from Angola, which he said was "too central to our interests" in southern Africa to ignore.