Abstract: Ultrasonic impact was conducted on the weld toe region of Q420qD/20MnMoNb thick plate dissimilar steel T-type welded joints with 18, 27 μm impact amplitudes, and the microstructure, residual stress and microhardness at the weld toe were analyzed under different impact amplitudes, and compared with those before ultrasonic impact. The results show that after ultrasonic impact, a plastic deformation layer, a lot of dislocations and a high compressive stress appeared on the surface layer of the weld toe, the grains were refined, and the hardness increased. When the ultrasonic impact amplitude increased from 18 μm to 27 μm, the depth of the plastic deformation layer increased from about 120 μm to 144 μm, the subcrystalline was further refined to about 25 nm in size, and the dislocation density increased to about 2.48×10¹⁴ m^-2; the residual compressive stress on the surface increased to about 433 MPa and its affected depth increased to about 1 410 μm; the surface hardness increased to about 400 HV and the depth of hardening layer increased to about 900 μm.