The effects of silicocalcium on the microstructure and mechanical properties of casting magnesium alloy Mg-6Al-0.5Mn(AM60) were studied. The results show that the microstructure of AM60 casting magnesium alloy is effectively refined by adding small amount of silicocalcium. The grain size decreases from 180 μm to 80 μm with 1.8% Si addition, while the size increases with 2.5% Si addition. The AM60+Si-Ca alloys mainly contain Mg matrix, β-Mg17Al12 phase, Al8Mn5 phase and a small polygonal type Mg2Si phase in matrix. Al8Mn5 can act as the heterogeneous nucleation for the Mg2Si phase. With the increase of silicocalcium, the content of Mg2Si phase increases gradually, the Mg2Si particles grow up and change coarse gradually. The microhardness of AM60 matrix increases with silicocalcium addition. The peak values of the tensile strength, elongation and impact toughness appear simultaneously with 1.8% silicocalcium addition, and the tensile strength, elongation and impact toughness are heightened respectively by 13.9%, 28.5% and 100%.