The influence of layer thickness on roughness, relative density, microstructure and mechnical properties of selective laser melting (SLM) 18Ni300 maraging steel were studied in this work. The results shows that when the laser power and hatching space are constant, the relative density first increases and then declines with the decrease of the laser volume energy density under the layer thickness less than or equal to 50 μm; whereas the relative density gradually declines with the decrease of the laser volume energy density under the layer thickness larger than or equal to 70 μm. At the same time, with the increase of the layer thickness and the decrease of the scanning velocity, the cooling rate gradually decrease during the SLM processing, thus the grain size gradully increase. Through the optimization of the SLM parameters, the dense (≥99.0%) 18Ni300 specimen can be obtained under all layer thicknesses 30~70 μm, the hardness, ultimate strengths and elongation gradually declines with the increase of the layer thickness, and the tensile fracture all shows the plastic fracture mechanism.