The effects of different high content of Ca and heat treatment on microstructure and mechanical properties of the as-cast Mg-6Al-5Zn-xCa-5Gd alloys were investigated in this paper. The results indicate that with the addition of Ca content in as-cast condition, the β-Mg25(Al, Zn)37.5 phase which continuously distributes in grain boundary becomes finer. The granular Al2Gd and the block CaZn5 phase gradually dissolve in the matrix more uniformly. Hence the tensile strength and yield strength increases firstly, then decreases, and the elongation decreases all the way. After the solid solution treatment, the most of β-Mg25(Al, Zn)37.5 phase is dissolved in the matrix, and the strength and the elongation increases obviously. With the increase of Ca content, much more granular and lump phase distribute in the surface. And the ultimate tensile strength and yield strength increases firstly, then decreases, but little change has happened with the elongation. After solid solution treatment and aging treatment, the age-hardening of alloys reachs the peak at 12h. Then the hardness tends towards stability after slightly lowering, which shows that the alloy has significant thermal stability. Fracture analysis shows that with the increase of calcium and the different heat treatment, fracture mechanism of the alloys transforms from brittle to ductile. With the content of Ca increasing to 6wt%, dimples can be observed obviously. After the heat treatment, the dimples are increasing and the size of which are uniform. The mechanical properties of the alloy of Mg-6Al-5Zn-6Ca-5Gd by solid solution treatment at 390℃/8h and aging treatment at 200℃/12h get optimal performance.