Abstract: The 18 Ni300 die steels were prepared by the optimized selective laser melting, and the effects of scanning speed and laser power on the mechanical properties of 18 Ni300 die steels were studied.The results show that, when the laser power is constant, the relative density and the comprehensive mechanical properties of 18 Ni300 die steel specimen increase first and then decrease with the increase of scanning speed.When the scanning speed is constant, the relative density and the comprehensive mechanical properties increase with the increase of laser power.When the laser energy density is about 150 J·mm^-3, the relative density of specimen is the highest.At the optimal scanning speed and laser power of 400 mm·s^-1 and 175 W, the relative density, tensile strength, microhardness, and elongation of specimen are 99.58%, 1101 MPa, HV 348.4, and 6.44%, respectively.