The effects of silicon and AlP on microstructure and properties of Mg-8Zn-4Al-0.3Mn alloy were studied. When 0.96%Si was added in the base alloy, Chinese script Mg2Si formed, the precipitation of τphase was restrained while the precipitation of φ phase was facilitated for decreasing of the concentration of solute in the residual liquid phase, at the same time, the fluidity was remarkably improved. To modify Chinese script type Mg2Si, a new green modificator--Al-AlP master alloy was added in Mg-8Zn-4Al-0.3Mn-1Si alloy. It showed that AlP addition promoted the formation of fine polygonal type Mg2Si particles by providing the nucleation sites, refined the grains, facilitated the precipitation of τphase while inhibited the precipitation of φ phase by changing the concentration of solute in the residual liquid phase and shortening the diffusion distance of the atoms. The improved microstructure of the modified alloy resulted in the large improvement in tensile properties at both ambient temperature and 150 as compared to the base alloy.