Abstract: In order to reduce welding residual stress, ultrasonic impact was applied on the back of the weld bead during welding process. The results show that, the in-situ treatment of ultrasonic impact can reduce residual stress effectively, the reduction effect of longitudinal residual stress is more obvious than that of transverse residual stress. When the ultrasonic impact is applied in the elastic-plastic transition temperature zone of the weld, the impact head diameter is relatively small and the impact head shape is spherical shape, the effect of reducing welding residual stress is better. The mechanism of in-situ ultrasonic impact is that, when the impact is applied on the back of the weld bead, tensile plastic deformation is produced at the face side of the weld bead. It counteracts more compressive plastic deformation that was generated during the heating expansion stage and finally leads to the reduction of the residual stress.