A federal judge yesterday reduced the sentence he had imposed on convicted kidnapper Thomas Edward Bethea from life imprisonment to a maximum of 30 years in prison.

Bethea is one of eight persons who have been convicted in the January 1976, kidnapping of Washington truck executive Alan Lewis Bortnick. The 27-year-old Bortnick was released unharmed after his father paid $250,000 in ransom.

U.S. District Judge Charles R. Richney reduced the sentenceafter prosecuters confirmed in court that Bethea had cooperated with the government in its investigation.

After Bethea's guilty plea and original sentence, two "insiders" who conceived and helped execute the kidnapping of the Square Deal Trucking Company executives were indicted and pleaded guilty.

Under the terms of the 30-year sentence imposed yesterday, Bethea can be released at any time the parole board sees fit. Under the life sentence, the 39-year-old Bethea would have had to spend a minimum of 10 years in prison before becoming eligible for parole.

Bethea received $105,000 of the ransom money, he has told prosecuters. He said he spent, lost or gambled away all but $12,000 of it while he was a fugitive for two months, and that the government recovered the remaining money from him after he was arrested.