Particle-reinforced aluminium-based composites are gaining attention for their excellent overall performance, and selective laser melting (SLM) provides a new path for the fabrication of these materials. SLM was used to form AlSi10Mg composites with different contents of B4 C particles. The effects of different forming process parameters (laser power, scanning speed and scanning spacing) on the microstructure, density and mechanical properties of the formed parts were studied. The results show that the B4 C/AlSi10Mg composites exhibit fewer internal voids and cracks, with a uniform distribution of B4 C particles in the aluminium matrix and good bonding between the B4 C particles and the AlSi10Mg matrix. The density of the 3 wt. %B4 C/AlSi10Mg composite material reaches 99.39% with a laser power of 240 W, a scanning speed of 1 400 mm/s, a powder thickness of 0.03 mm and a scanning spacing of 0.13 mm. When the B4 C particle content is increased to 5 wt. %, although the hardness is improved, the density, tensile strength and elongation are reduced.