This study investigates the effect of process parameters on the relative density and microscopic pores of samples formed using selective laser melting (SLM) technology. To analyze and discuss the 18Ni-300 maraging steel, an orthogonal experimental scheme is designed. With the three-dimensional SLM forming process parameters (laser power, scanning speed, and hatch spacing), the relative density of printed parts is studied and analyzed based on the gray theory, the corresponding relationship between the relative density and volume energy density is obtained, and the microstructure and holes of the material are analyzed. The results show that when the volume energy density is 30‒50 J/mm³, the relative density is low. In addition, increasing the volume energy density within this range can significantly improve the relative density of the material and keeping the volume energy density at 50‒150 J/mm³ can achieve high relative density. With increasing relative density, the holes in the sample gradually becomes smaller and its shape becomes regular. When the relative density reaches 99%, the tiny holes with a regular shape are obtained. The samples with high or low density can be obtained by changing the range of process parameters.