A federal jury ruled today that the common-law wife of a man killed by Prince George's County police in 1967 is the man's lawful widow and not a second woman who married him subsequently in a ceremony.

In favoring Jacqueline E. Jones of Silver Spring over Betty Ann Harris of Sharon, Pa., the six-member civil jury apparently accepted Jones' argument that her relationship with William C. Harris met the standards of common-law marriage in Ohio, where the couple first lived, and that his second marriage was thus void as bigamous.

The two women vied during three weeks of testimony in U.S. District Court here for designation as the rightful widow in their bid for part of $9 million in civil claims against Prince George's County and several county law enforcement officials in the fatal shooting of Harris.

The jury's action sets the stage for what promises to be a lengthy trial over whether the police wrongfully shot and killed Harris during a series of convenience store robberies in 1967.

Jones, 37, survivors of a second man police killed, and two other men arrested during the robberies contend that a group known in police circles as the "death squad" arranged the holdups, with officers directing street informants to recruit participants.

Police staked out targeted convenience stores and shot or arrested the participants when they showed up. Lawyers for the survivors contend that the holdups were approved by Prince George's County State's Attorney Arthur A. Marshall Jr. and that police records of the incidents later were falsified.

Marshall and other county officials deny staging the robberies. They say that the shootings and arrests occurred after informants told them of planned holdups and that officers routinely staked out the locations.

Though the "death squad" incidents occurred 15 years ago, they did not come to public attention until 1979, when The Washington Post published stories containing the allegations. The lawsuit filed by Jones and the others, which contend that their 14th amendment rights to due process were violated, followed.

In the preliminary clash over lawful widowhood, Betty Ann Harris contended that Harris did not have a bona fide common-law relationship with Jacqueline Jones, but lived essentially as a bachelor, moving from woman to woman.

Jones argued that she and Harris lived together after agreeing to become man and wife and were perceived in the community as a married couple--all requirements of common-law marriage in Ohio.