Dissimilar metal laser welding of cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft was studied. Microstructure of the welded seam was characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Results showed that side-blow shielding gas was warped in the melt pool with excessive flow rate of shielding gas by progressively reducing of heat input of laser. Bubble defect of seam ending can be inhibited by optimizing flow rate of side-blow shielding gas. Microstructure of the weld and base metal interface is mainly composed of dendrites whose growing direction is nearly parallel to the negative heat-transfer direction. Distribution of Laves phase have discrepancy in different region of the welded seam because of different mixing proportion of K418 and 42CrMo,and different solidifying velocity in different region of the melting pool.