S355J2G4 steel MAG welded joints were ultrasonic impacted under different times, and the influence of ultrasonic impact time on surface morphology and wear resistance were systematically investigated. The results indicated that ultrasonic impact could significantly refine the surface microstructure as well as generate a surface plastic deformation layer. As ultrasonic impact time extended, the surface microstructure size decreased and plastic deformation layer thickness increased. However, too long ultrasonic impact time would form some defects near the weld toe. The surface weight loss and friction coefficient both significantly decreased as the impact time increased, which indicated that the wear resistance of weld surface elevated when ultrasonic impact time increased. The ultrasonic impact had the most optimal influence when the impact time was 20 min.