In order to solve the problem of fatigue crack of metro vehicle body and bogie frame, the mechanism and feasibility of ultrasonic impact technology to enhance the fatigue strength of welded structure were explored. In order to obtain the strengthening effect of different treatment methods, the welded butt joints of SMA490BW steel were treated by mechanical grinding and ultrasonic impact with different process parameters. The stress distribution, joint morphology, grain structure, fracture morphology and fatigue life of the specimens were analyzed by means of stress analyzer, metallographic microscope, ultrasonic fatigue machine and scanning electron microscope. The results show that ultrasonic impact can effectively improve the stress concentration of butt joint, refine the surface grain and adjust the distribution of surface stress field, and transform the welding residual tensile stress into larger residual compressive stress. Under the impact of 1.5 A/10 min process parameters, the maximum average life of the sample is 3.35×107 cycles, which is 3.8 times of the mechanical grinding sample and 10 times of the original sample, and the fatigue fracture failure mode remains unchanged. The results show that ultrasonic peening can significantly improve the fatigue life of welded structure by surface modification, and it is better than mechanical grinding method. The ultrasonic impact method is highly suitable for the needs of vehicle crack repair, but the relevant standards, equipment and technology are not perfect. It is suggested that the ultrasonic impact method be further explored and finally incorporated into the subway vehicle crack repair process.