The effects of second phases on microstructure and mechanical properties of as-cast Mg-Ca-Sn permanent moulding magnesium alloy were investigated. The results indicate that sole addition of Ca and Sn refined grain size of as-cast magnesium alloy in a certain degree. Mg2Ca phase precipitated at grain boundaries continuously and granular Mg2Sn phase existed at grain boundaries and inside the grains. Point and needle-like CaMgSn phase can be formed after Ca and Sn combined addition. Because CaMgSn particles acted as heterogeneous nucleus of α-Mg during solidification, the grain of α-Mg was significantly refiner than sole addition one. The edge-to-edge matching model was used to analyze the crystallography mismatch relationship between CaMgSn and α-Mg. Microhardness of Mg-Ca-Sn magnesium alloy was improved remarkably with finer grain size.