S355J2G4 steel joints were post-treated with ultrasonic impact at different times. The influences of impact time on microhardness, tensile strength and fatigue property were investigated. The microhardness of the weld surface significantly increased when the impact time lengthened, and the increasing depth of the microhardness was roughly the same as the plastic deformation layer thickness. Ultrasonic impact time did not have obvious influence on the tensile properties. The fatigue life increased first and then decreased as the impact time lengthened, and new defects appeared near the weld toe under the excessive impact time was the primary cause of fatigue life reduction. Comprehensively considering the microhardness, tensile strength and fatigue property, 20 min was the ideal ultrasonic impact time.