The common-law wife of Ronald Louis Humphrey, the United States Information Agency employee accused of being a Vietnamese spy, has a 16-year-old son who is being held "a hostage" in Communist Vietnam. Humphrey's lawyer said yesterday.

Four other children and the woman herself got out of Vietnam this fall only after the intercession of German novelist and Nobel laureate Henrich Boll, and diplomats from Sweden, Germany and the United States, according to the lawyer.

Humphrey's common-law wife, Chieu Thi Nguyen, was held captive for 225 days by the Communists after the fail of Saigon, according to a statement she made to immigration officials. She was imprisoned and tortured, she said, because she had "worked for the U.S. government," according to the immigration official and Humphrey's lawyer.

The statement, released by A. Andrew Giangreco, Humphrey's court-appointeed lawyer, was the first indication of the pressures the 42-year-old foreign service officer may have been under at the time he is accused of having passed classified government documents to a network of spies.

Giangreco refused to speculate no the impact Humphrey's family may have had on his actions, but Arlington neighbors said that from the time the middle-level USIA evaluations officer rented the red brick house at 618 S. Irving St., he seemed very concerned about reuniting his family.

"All he could talk about was getting his family here," said Linda Andrews, who lives across the street from Humphrey.

In a statement yesterday, Humphrey, who is being held in an Alexandria jail, said that Boll and the Swedish and West German ambassadors to Hanoi had worked to free the members of the Nguyen family. They were joined by officials of the U.S. Consulate in Frankfort, near where Humphrey worked as head of America House, a USIA-run library and community center, the statement said.

State Department officials declined to comment on the statement yesterday, but a spokesman for the Swedish embassy in Washington acknowledged that his country "for humanitarian grounds" had helped the family.

Pressure to help came from Humphrey's "friends and contacts in western Germany" and Boll, said embassy spokesman Lars Arno. "We had no stake in it," he said. "We acted on humanitarian grounds to help keep families together."

According to friends of Humphrey, Nguyen, whose husband was killed as he fought in Saigon's army, managed to get ouut of Vietnam after her imprisonment. She returned in 1977, however, to get out the four children. Two of the children are offspring of her marriage to the Army officer and the other two are children of relatives, who were killed during the war, neighbors said.

According to Immigration spokesman Verne Jervis, the woman and the four children left Vietnam on July 21, 1977, and arrived in New York on Nov. 24 under a "conditional entry" permit issued previously by the U.S. consulate in Frankfort.

She was imprisoned after she failed to catch a departing boat when Saigon fell in 1975, according to her lawyer. She had told friends she was tortured during her imprisonment and her hair was pulled out.

Humphrey apparently met Nguyen while he was assigned to Vietnam from 1969 to 1970, an assignment that one associate said left a deep impression on Humphrey. "Most USIA guys continue to follow events in a country after they have served there . . . But, his was not the standard kind of feeling," he associate said. "There was a sense that (Vietnam) ment more to him. He was concerned about it."

Humphrey continued to have a strong interest in Vietnam long after he had left there and been reassigned to Germany, the associate said.

His assignment there, which ended in 1976, won him high marks from his superiors and may have helped him meet Boll, who lives near Cologne and has frequently been supported liberal causes in Europe. Boll was the host for Russian writer Alexsandr Solzhenitsyn after he was expelled from the Soviet Union in 1974.

The statement and comments by Giangreco were the most detailed to date about Humphrey and the woman whom he had identified in court records only as "Kim," 32.

Humphrey was arrested Tuesday with David Truong, also known as Truong Dinh Hung, the 32-year-old son of a Vietnamese politician who ran for president of Vietnam against Nguyen Van Thieu in 1967. After he lost the election, Truong's father was imprisoned for five years. Until his arrest Tuesday, the younger Truong worked as a $9,000 a year mailroom superintendent in Washington, while studying economics at George Washington University.

Both Truong and Humphrey have pleaded innocent to charges of espionage, theft of government documents, failure to register as foreign agents and other crimes. If convicted, they could each face life imprisonment.

Humphrey shouted aloud his defense to reporters as he was being led handcuffed into an Alexandria courthouse this week. At that time, Humphrey, the first USIA officer ever indicted, professed his innocence and said he had "never suspected him." He did not elaborate on who "him" referred to.

Truong has issued no statement since his arrest.