In this paper, the aluminum alloy (AlSi10Mg) additive manufacturing by selective laser melting (SLM) was used for laser welding experiments, and then the effects of laser power on microstructure and mechanical properties of AlSi10Mg alloy welding seam by SLM additive manufacturing were studied by OM, SEM and EDS technical analysis, and mechanical properties testing. The results show that the width of the laser-selective melting heat affected zone is small. The weld zone is columnar dendrite structure. With the increase of laser welding power, the laser energy increases, the melting depth increases, the hardness of the weld zone and the tensile strength of the joint decrease, and the elongation is low, (4.1%, 3.4%, and 3.1%, respectively). The primary dendrite spacing and microstructure in the weld zone gradually increase, and Si element is easier to aggregate in the alloy, leading to the fracture separation of reticular Al-Si eutectic, while Mg element content is small, and there is little change. The pores are distributed in the welding seam of laser welding, and the pores are larger towards the surface. Because the pores reduce the effective bearing area of the cross section, and the local stress concentration accelerates the crack propagation, it is very important to reduce or avoid the pores in laser welding. Finally, the fracture section of the joint shows the appearance of a river pattern with the characteristics of brittle fracture.