Abstract: The residual keyhole after friction stir welding can reduce the mechanical properties of the joint, and the repairing of the keyhole has an important engineering significance. Taking AZ31B magnesium alloy as the object, the effect of adding ultrasonic on the passive filling friction stir reparing（PFFSR） process was studied. The results show that compared with the process without ultrasonic, the flash is reduced, material flow is increased and atomic diffusion is strengthened by ultrasonic, the defect-free joint is obtained; the tensile strength of the joint with ultrasonic assisted PFFSR reaches 182.9 MPa,which is increased by 38% compared with that of the joint without ultrasonic. Moreover, the fracture morphology of ultrasonic assisted PFFSR joint has a large number of dimples, which is typical ductile fracture.