In this paper, the effects of substrate preheating temperature on the microstructure and mechanical properties of AlSi9Mg1ScZr alloy samples prepared by selective laser melting were investigated. SLM samples were prepared at three different substrate preheating temperatures of 35°C, 85°C and 135°C and subjected to microstructure observation and property testing, respectively. The results showed that the substrate preheating temperature set to 135℃ caused the in-situ aging effect of the alloy during the printing process due to the combined effect of the substrate preheating temperature and laser scanning heat input, which promoted the precipitation of elements from the supersaturated solid solution while retaining fine dendrites and Si lattices. Compared to the sample with the substrate preheated at 35 °C, the nanoscale Mg₂Si and Si phases precipitated significantly more in the α-Al matrix and dendrite boundaries, which served to improve the strength; however, the precipitation of the micron-scale Fe-rich phase had a negative effect on plasticity. At a substrate preheating temperature setting of 135°C, the prepared AlSi9Mg1ScZr alloy exhibited a yield strength of 360 MPa, a tensile strength of 502 MPa, and an elongation of 7% in the 0° direction, and a yield strength of 331 MPa, a tensile strength of 511 MPa, and an elongation of 5.4% in the 90° direction. In this study, the microstructure of AlSi9Mg1ScZr alloy prepared by SLM was improved by increasing the substrate preheating temperature and achieving in situ aging of the SLM samples during SLM, which significantly reduced the residual stresses and obtained ultra-high strength AlSi9Mg1ScZr alloy samples without subsequent heat treatment .