The effects of stibium element on microscopic structure and mechanical properties of Mg-5Sn-2Al-1.5Zn-0.8Si magnesium alloy were studied when Sb was added into melt in the form of pure metal. The results indicate that Sb element and Mg matrix combine to form Mg3Sb2 phases. The addition of 0.9% Sb has a strong modification for Chinese script morphology of Mg2Si phases. Sn can replace Si of Mg2Si phase to form Mg2(Si,Sn) composite phases, whose physical properties lie between Mg2Si and Mg2Sn. With the increase of Sb content, the elongations of the as-cast alloy and the extruded gradually decrease, while tensile strengths increase firstly and then decrease. Strength and plasticity of the extruded alloy are better than that of the cast alloy, and the increase of Sb content is beneficial to improve the heat resistance of Mg-5Sn-2Al-1.5Zn-0.8Si alloy