<p>Maraging steels are widely used in aerospace, mold manufacturing, and high-end equipment manufacturing due to their high strength, good toughness, and excellent processability. Selective laser melting (SLM), a common metal additive manufacturing (3D printing) technology, offers a novel approach to fabricating maraging steel components with complex geometries. However, the SLM process parameters significantly influence the final microstructure and properties of the formed parts. This study systematically investigated the effects of key process parameters, including laser power, scanning speed, layer thickness, and volumetric energy density (VED), on the microstructure, mechanical properties, and defect formation of 18Ni300 maraging steel. The results show that optimizing the VED to 107.78 J/mm³ can synergistically improve molten pool stability, suppress defects such as lack of fusion and porosity, and refine grains. Consequently, the material achieves a relative density of 8.06 g/cm³, a peak impact toughness of 75.2 J/cm², and high yield strength, realizing an excellent strength–toughness balance. Optimizing VED can significantly enhance material density, refine the grain microstructure, and improve overall mechanical properties. This study provides a theoretical basis and practical guidance for parameter optimization in the SLM processing of maraging steels.</p>