Douglas J. Bennet, assitant secretary of state for congressional relations, has been chosen to become the new director of the Agency for International Development, Carter administration sources revealed yesterday.

Bennet, 44, would succeed former Ohio governor John J. Gilligan, who was pressured to resign from the AID job in February. Bennet's appointment, to be formally announced soon, is subject to Senate confirmation.

A former staff director of the Senate Budget Committee and aide to the late Hubert H. Humphrey, Sens. Thomas F. Eagleton (D-Mo.) and Abraham A. Ribicoff (D-Conn.), Bennet is extremely popular on Capitol Hill, and has been credited with an instrumental role in many of the administration's foreign policy successes in Congress.

Several administration aides said Bennet's departure from his State Department job on the eve of the Senate debate on the new strategic arms limitation treaty would be a serious loss to President Carter's lobbying team. Secretary of State Cyrus R. Vance has not yet settled on a successor to Bennet at State, sources said.

Gilligan was forced out of the AID directorship after a series of disputes with Vance and the White House. During the aid agency, which has 5,800 employes, Gilligan emphasized getting more of the staff into the field.

Gilligan also pressed for a reorganization plan that would have put all foreign aid and related activities under a single bureaucratic roof. The administration has now adopted a partial version of this reorganization, proposing creation of a new International Development Cooperation Administration.