The effects of Al-P addition on the microstructure and mechanical properties of as-cast Mg-5%Sn-1.25%Si magnesium alloy were investigated. The results show that the phases of the as-cast alloy are composed of α-Mg, Mg₂Sn, Mg₂Si, little P, and AlP. The Chinese character shape Mg₂Si phase changes into a granular morphology by P addition because AlP can act as a heterogeneous nucleation core for the Mg₂Si phase. When 0.225wt% of Al-3.5%P alloy is added, the mechanical properties of the Mg-5%Sn-1.25%Si alloy are greatly improved, and the tensile strength increases from 156 to 191 MPa, an increase of 22.4% compared to the alloy without P addition. When the amount of Al-3.5%P reaches 0.300wt%, a segregation phenomenon occurs in the granular Mg₂Si phase, and the tensile strength and hardness decrease though the elongation increases.