Ronald L. Humphrey, 42, the former U.S. Information Agency employe convicted last May of spying for Vietnam, was set free on $75,000 bond yesterday by a federal judge in Alexandria.

Humphrey, who has been in custody since his arrest Jan. 31, appeared in a two-minute hearing before U.S. District Court Judge Albert V. Bryan during which his release papers were signed. Then his handcuffs were removed and shortly afterward he was reunited with his parents and friends.

"It's a very happy day for me," the soft-spoken Humphrey told a group of reporters. "I'm not a spy . . . Every cell in my body tells me there's nothing I would ever do to betray my country."

Standing outside the federal courthouse in Alexandria, Humphrey squeezed and patted his mother's hand and held tightly the hand of Kim, the Vietnamese woman with whom he had lived before his arrest.

While Kim's three daughters, dressed in look-alike pink dresses, and her 14-year-old son looked on, Humphrey spoke of his yearning to "feel the rain, see a star, touch a tree" and to "see the sun." He has been held in the Alexandria City Jail since his arrest.

It was for Kim, who Humphrey said saved his life in Vietnam, and her children that Humphrey said he took scores of government documents from his USIA office and gave them to Capitol Hill lobbyist David Truong. Truong was accused of passing the documents to Vietnamese Communists in Paris. Both Humphrey and Truong were convicted of espionage and sentenced to 15 years imprisonment.

Truong, the son of a once-prominent Vietnamese politician, had been able to raise his $250,000 bond before his trial. He remained free after the trial pending appeal of his conviction.

On Sept. 18 Judge Bryan reduced Humphrey's $150,000 bond by half because, he said, it was unfair for Truong to be free when Humphrey was still in custody.

According to the Rev. George W. Hill, pastor of Calvary Baptist Church in Washington, Humphrey's release was secured by contributions of $20, 000 from the board of national ministries of the American Baptist Churches, $15,000 by the American Friends Service Committee, $11,750 by the Prison Fellowship, a group founded by Watergate figure Charles Colson, and individual donations.

Both men have appealed their convictions to the U.S. 4th Circuit Court of Appeals in Richmond.

Humphrey said that he is unsure of his future, but is sure he must find a job. "I have four children to feed. I'm going to have to do something," he said.