A high Mg-content Al-Si-Mg3 aluminium alloy was designed for SLM based on the technical characteristics of the liquid quenching in SLM. The effect of the process parameters and aging treatment on the microstructure and the hardness of the SLM-formed AlSiMg1.5 alloy were systematically studied. The results show that all SLM-formed samples are composed of α-Al, Si and Mg2Si phases. High laser energy density is beneficial to increase the formability of the samples. The minimum porosity ~ 0.07% is obtained for the sample fabrication at a laser power of 160 W with a scanning speed of 200 mm/s. The Mg element dissolved in α-Al and the proportion of Si-rich microstructures increases with the increase of the scanning speed, as a result, the hardness of the SLM-formed samples increases gradually. The maximum hardness of the SLM-formed sample reaches 194±3 HV. The hardness increases obviously after aging treatment the SLM-formed samples at 150 ℃ for different times due to the precipitation of nanoparticles. The maximum hardness of the samples is 210 ± 2 HV, which is much higher than the reported SLM-formed Al-Si and Al-Si-Mg alloys. A special Al-Si-Mg alloy for SLM with excellent formability and mechanical properties was reported in this paper.