Abstract: After welding dissimilar steel by laser swing welding, the equilibrium phase diagrams of base 3Cr13 and VG10 were calculated by JMATPro software, and the phase composition and microstructure of weld, fusion zone and heat affected zone were analyzed by XRD, SEM, EPMA and other techniques. in addition, the microhardness distribution of welded joints was determined. The results show that the weld is mainly composed of phases and carbides M₇C₃, and from the fusion line to the weld center, the structure changes from plane crystal to cellular crystal, cellular dendrite, columnar crystal, dendritic crystal and equiaxed crystal. The microstructure of the weld exists micro-segregation, in which C and Cr elements are enriched at the grain boundary, Fe elements are enriched within the grains, and bar-like M₇C₃ precipitates at the grain boundaries. There is a C migration phenomenon at the base material of the fusion line near. Among them, acicular martensitic structure is generated on the 3Cr13 side, and the unmixed zone exists in the fusion zone on the VG10 side. At this position, the massive, island tissue is embedded in the base Material, and there are lamellar carbides on the structure. The hardness of the base metal on both sides of the fusion line is the largest, and the hardness of the weld zone changes little. The hardness of the heat affected zone decreases with the increase of the distance away from the weld center.