Thick plate dissimilar steel welded structure used in bridge engineering, in the welding process due to the different coefficients of thermal expansion between dissimilar metals and thick plate multi-channel welding brought about by a number of thermal cycling and the thermal effect of different passes, will lead to high residual tensile stress in the weld and heat-affected zone, seriously threatening the safety of the welded structure. Ultrasonic impact is a surface strengthening technology. When the ultrasound driven impact needle rapidly impacts on the surface of the workpiece, intense plastic deformation will be produced, so residual compressive stress is introduced to promote the service of the structure. In this work, for the Q420/ZG300 thick plate dissimilar steel welded test plate, a combination of experimental and numerical simulation was used to investigate the effect of ultrasonic impact treatment on the improvement of the residual stress field in the surface layer of such structures after welding. Firstly, the temperature-displacement sequential coupling model of thick plate dissimilar steel was established based on the thermoelastic-plastic finite element method, and the welding residual stress field was simulated by simplifying and merging the welding channel and adopting the instantaneous heat source model for the difficulties of convergence and long computation time in the simulation of thick plate welding. Then, the three-dimensional display dynamics ultrasonic impact model was established, and the welding residual stress was input to investigate the effect of different ultrasonic impact processes on the residual stress in the surface layer of the transition zone. The residual stress distribution of the welded test plate and the impacted specimen was tested with a residual stress meter. There is high residual tensile stress in the weld and heat-affected zone after welding, with the peak longitudinal residual stresss of 467 MPa and the peak transverse residual stress of 245 MPa. In the region far from the weld, there is low tensile stress or residual compressive stress, making the test plate in a self-equilibrium state. After ultrasonic impact treatment, the residual tensile stress in a certain depth range of the surface layer of the transition zone is transformed into compressive stress, and the maximum compressive stress occurs in the surface layer or sub-surface layer, and with the increase of the depth, the compressive stress decreases until the tensile stress appears. The maximum longitudinal residual compressive stress is 333 MPa and the maximum transverse residual compressive stress is 321 MPa when the amplitude is 15 μm and the diameter of the impact needle is 4 mm. The stress decrease is 636 MPa and 507 MPa, and the depth of influence of the compressive stress is 0.94 mm and 0.96 mm, respectively. When the amplitude and the diameter of the impact needle increase, the depth of influence also increases. When the spacing of impact needles increases, the depth of influence decreases and then increases, but the increase is not obvious. The experimental measurements have a good correspondence trend with the simulated values, and the results of this study can provide guidance for the application of the ultrasonic impact process method in this kind of welded structures.