The urgent need for lightweight, high-accuracy, and personalized products has led to the rapid development of additive manufacturing. Selective laser melting (SLM), which is a very promising additive manufacturing technique, has attracted remarkable attention. The mechanical properties of SLM parts are highly related to the formation of pores and cracks. In this work, SLM parameters for AlSi10Mg alloy were optimized, and the SLM AlSi10Mg sample with a high relative density of 99.63% was obtained. The SLM sample exhibited good properties, including an ultimate tensile strength (UTS) of approximately 478 MPa, a total elongation of 8%, and an average hardness of 122 HV along the horizontal direction. However, due to a high cooling rate, an inhomogeneous microstructure with refined grains and a Si network was obtained. To achieve a homogeneous microstructure and further improve the elongation of the SLM samples, the effect of heat treatments on the microstructure and mechanical properties of the SLM samples along the horizontal direction was analyzed. After the heat treatments, the strength of the samples changed significantly and the elongation was significantly improved. Further, after a solid solution treatment at 540oC for 1 h, the UTS significantly decreased to approximately 246 MPa and the elongation increased to more than 22%. For the sample annealed at 236oC for 10 h, a UTS of approximately 368 MPa and elongation of approximately 17% were obtained. Moreover, the sample subjected to ageing at 130oC for 4 h exhibited a high strength similar to the level of the SLM sample, the elongation was increased to approximately 11.9%, and the hardness was approximately 133 HV which is 10% higher than that of the SLM sample. The improved performance of the aged samples can be attributed to the combination of solution strengthening, microstructural refinement, and precipitation strengthening. The results show that low-temperature ageing is the optimized heat-treatment method for SLM samples with fine microstructures.