A U.S. District judge sentenced an aide to former CIA agent Edwin P. Wilson to six months in prison, three years on probation and a $10,000 fine yesterday for his role in helping Wilson ship explosives to Libya.

Judge John H. Pratt said Douglas M. Schlachter's activities were "not something we can kiss off lightly."

But Pratt said Schlachter, who contended that he thought Wilson was working for the Central Intelligence Agency, was "for some time not convinced that he was operating outside the protection of the government," until the very late stages.

Schlachter, 39, who was indicted with Wilson in August, 1981 on four charges of conspiracy to export munitions and exporting munitions without a license, returned voluntarily in November, 1981 from his home in Burundi to plead guilty to two of the charges.

Pratt heard most of the arguments prior to sentencing in an hour-long closed hearing with government prosecutors and Schlachter's lawyer, Alvin Askew. At a brief public hearing, Pratt said the secrecy was necessary because of ongoing investigations involving Wilson and his activities. Wilson was convicted in Alexandria in November of exporting weapons to Libya and sentenced to 15 years in prison.

Wilson is scheduled to stand trial in Houston on Jan. 24 on charges that he shipped 20 tons of plastic explosives to Libya, and faces a subsequent trial in Washington on other charges, including one that he conspired to assassinate a prominent Libyan exile.

Schlachter has been cooperating with federal prosecutors in various investigations of Wilson's alleged activities, Pratt noted during the hearing yesterday.

After the hearing, Schlachter--who began working for Wilson in 1971, while Wilson was still an agent with the CIA--issued a statement through his attorney saying that "from the beginning of my employment with Edwin Wilson . . . I always believed I was acting for an official U.S. intelligence operation."

The CIA consistently has denied that it either knew of or condoned Wilson's Libyan operation.