Abstract: Mg-4Li-1Ca-4.1Al-0.5Si alloy was prepared by ordinary gravity casting method. The microstructure evolution and microhardness of the alloy after solution treatment at different temperatures for 1 h and 400 ℃ for different time were investigated. The results show that the cast alloy consists of α-Mg matrix,（α-Mg+AlLi） binary eutectic,（α-Mg+AlLi+（Mg,Al）₂Ca） ternary eutectic and bulk CaMgSi. Among them, the AlLi phase is in the form of fine layer and distributed along the grain boundary.（Mg, Al）₂Ca is distributed in the center of the AlLi layer, and together with the matrix α-Mg forms a ternary eutectic. When the alloy is solution heat treated for 1h, the AlLi phase decomposes at 350 ℃ to form small particles, and（Mg,Al）₂Ca phase forms particles or needle-like fibers. At 400 ℃, the（Mg, Al）₂Ca eutectic phase dissolves in a large amount, and the amount is drastically reduced, while the AlLi phase grows slightly. After solution treatment at 400 ℃ for 5 h, the number of（Mg, Al）₂Ca eutectic particles increases sharply and begin to segregate and grow. At the same time, AlLi dissolves again to form nanosolute particles, which are uniformly dispersed in the matrix. The hardness of the alloy after treatment at 350 ℃ for 1 h is the highest, which is 105.3 HV.