In this study, the Ti6Al4V alloys were fabricated by selective laser melting (SLM) technology. The effect of laser energy density (LED) on the relative density, micorhardness, compression strength and plasticity of the Ti6Al4V alloys was systematically studied. The optimum LED processing window for SLM of Ti6Al4V alloys was in the range of 84.8–163.6 J/mm_³. In the optimum LED window, the TiN reinforced Ti6Al4V matrix composites were fabricated by SLM in different N₂ concentration (3 vol.%, 10 vol.% and 30 vol.%) atmospheres based on gas–liquid reaction. The principle of the novel technology for fabricating of the composites is as follows: decomposition of N₂ near the Ti6Al4V melt pool generates N atoms/ions, the gaseous N atoms/ions react with liquid Ti atoms to in-situ synthesize uniformly distributed TiN reinforcements which combine with the Ti6Al4V matrix during melting–solidification, finally, the TiN reinforced Ti6Al4V matrix composites are built by SLM layer-by-layer. Effect of N₂ concentration on the microstructure and mechanical properties of the Ti6Al4V matrix composites was researched. The composite fabricated by SLM in 3 vol.% N₂ atmosphere exhibited a good combination of high strength and high plasticity. The strengthening and toughening mechanisms were studied.