Abstract: 18-Ni300 martensitic steel was prepared by laser selective melting. The effect of laser power on the microstructure and properties of 18ni-300 martensitic steel was studied. The results show that with the increase of laser power, the grain size of cladding layer becomes smaller and the microstructure is dense; The side hardness of the sample is higher than that of the upper surface, and the tensile strength and elongation increase first and then decrease. When the laser power is 300W and the scanning speed is 1000mm / s, the comprehensive mechanical properties of the sample are the best: the tensile strength is 1217MPa, Vickers hardness is 377HV , and elongation at break is 8.93%. Its EBSD shows that in XOZ plane and YOZ plane, columnar crystals grow along the growth direction (z direction), the grains show weak texture, and there are many small angle grain boundaries, and the grain size is less than 10 μ m. It reflects the excellent mechanical properties of martensitic stainless steel to a certain extent. This study can provide reference for the reasonable selection of selected laser melting process parameters of martensitic aging steel.