Abstract: To explore the influence of solution treatment time on the microstructure and properties of Mg-Al-Mn-Zn-Si alloys, Mg-10Al-0.2Zn-0.35Mn-0.2Si alloy was used as the research object. The mechanical properties and microstructure changes after solution treatment at 425 ℃ for 5 to 60 h were investigated by OM, SEM, EDS and mechanical property tests. The results show that with the extension of solution treatment time, the grain size of the alloy exhibits an overall increasing trend, and obvious mixed crystal structure appears after 45 h; when the solution treatment time is 25 h, the alloy has the best comprehensive mechanical properties, with a tensile strength of 291 MPa and an elongation of 22.3%. Under the condition of long-time solution treatment (45 h-60 h), although abnormal grain growth occurs, the alloy still maintains high mechanical properties due to the synergistic effect of second phase strengthening, solid solution strengthening and mixed crystal structure. This study provides an experimental basis for the optimization of the heat treatment process of such alloys.