To improve its comprehensive properties, the W6Mo5Cr4VZ (MZ) high speed steel tool surface was nitrided by high power semiconductor laser. The differences of nitriding pool and surface microstructure undergone pure nitrogen and nitrogen-argon mixture gas were compared. The changes of the phase composition, microstructure and hardness of nitriding layer under different nitriding processes were analyzed. Results show that dendrites become fewer in the AlN nitride layer by increasing the ratio of argon, and the original large dendrites are gradually transformed into shorter equiaaxial crystal structures. AlN and Al are formed at the depth of 150~300 μm, and Al is the main phase at the depth of 450~600 μm. When the ratio of nitrogen and argon is between 3∶1 and 1∶3, the surface hardness reaches 900~1 000 HV, and the hardness of nitriding layer decreases slowly. As the proportion of argon increases gradually, the proportion of nitrogen decreases and less AlN dendrites are formed. With the ratio of argon rising, the nitrogen content in the molten pool decreases and the degree of undercooling increases, which makes the primary and secondary crystal axes longer, and reduces the radius of curvature at the dendrite tip. This study is helpful to improve the microstructure of laser nitriding layer of MZ tool, which can be extended to other materials with high surface requirements.