Abstract: In this paper, 18Ni300 mold steel was formed by selective laser melting (SLM), based on orthogonal test design, with relative density and microhardness as the optimization indexes, and the SLM forming process parameters were optimized by multi-factor polar difference analysis. The influence of key process parameters (laser power, scanning speed, scanning spacing) on the forming quality (relative density, microhardness and mechanical properties) of 18Ni300 mold steel formed by selective laser melting (SLM) was systematically studied. A central composite design experiment of the response surface was developed using laser power, scanning speed, and scanning spacing as process parameters, and density and hardness as response targets, verifying the accuracy of the orthogonal test. The experimental results show that when the process parameters are laser power 335 W, scanning speed 860 mm·s1, and scan spacing 90 μm, the comprehensive performance of SLM-formed 18Ni300 mold steel specimens is the best, with a relative density of 99.40% and a microhardness of 382 HV1. Further verification tests confirmed that the tensile strength of the samples reached 1258 MPa, the yield strength reached 1166 MPa, and the elongation after breaking was 6.0%. Microstructure observation showed that the specimen had dense tissue and few defects such as internal holes and cracks.