The 5CrNi4Mo die steel is successfully prepared by selective laser melting (SLM) additive manufacturing technology. The mechanism of the phase transformation is investigated and the influence of the applied laser linear energy density h (the ratio of laser power to scan velocity) on densification, microstructure, and mechanical properties of SLM-processed parts is studied. It shows that a high h results in the formation of residual pores and lowers the densification level, caused by severe balling phenomenon. A low h causes the limited wetting ability and low densification level. When h is optimized at 258.3 J/m, the SLM-processed parts have an improved densification level of 98.12% without apparent process defects. The laser-induced large solidification rate leads to the martensitic transformation. Alloying elements such as Mn, Ni, and Cr in the original powder can stabilize the undercooling austenite and lower the critical cooling rate of martensite transformation, ensuring the successful process of martensite transformation. The microstructures of martensite are further refined with decreasing h. When h is 193.8 J/m , high microhardness of 689.5 HV0.2, low friction coefficient of 0.44 and wear rate of 2.3×10^-5 mm³/(N?m) are obtained for the SLM-processed parts.