This paper clarifies the effect of laser wire filler welding process parameters of dissimilar steel on weld formation. Using 2 mm 45 steel of unequal thickness and 6 mm 316L stainless steel as the test materials, the laser wire filling welding method was used for welding, and the welding seam formation and the mechanical properties of the welded joint were studied with different butt gaps. The results show that with the increase of the butt gap, the residual height gradually decreases, and the weld morphology transitions from nail-shaped to H-shaped. As the butt gap increases, defects are formed in the weld. The hardness of the weld center decreases with the increase of the butt gap, and the highest hardness of the welded joint is located in the heat-affected zone of 45 steel. The tensile strength of the welded joint firstly increases and then decreases with the increase of the butt gap. When the butt gap is 0.6 mm, the surface morphology is good and the tensile strength is the highest, and the fracture position of the welding test is on the 2mm45 steel side.