AZ91 magnesium alloys with different Ce content were prepared, and the distribution of alloy elements and compounds structure were studied on the basis of analyzing the microstructure of the alloy, in order to investigate the reason of the change of the structure of the alloy. The results show that The solid solution of Ce in a -Mg is only a few dozen ppm, and the amount of solid solution of Al in a -Mg is less than the average content in the alloy, while Zn mainly exists in the β-Mg17Al12. The grain size decreases with increasing the Ce content, and the Ce is gather in front of solid/liquid interface lead to form composition under-cooling which cause the grain refinement. Ce can reduce the amount of Al participated in the eutectic reaction which make β-Mg17Al12 decrease and its distribution from continuous to discrete. Zn ion replacement Al ion results in lattice distortion of β-Mg17Al12 and the lattice constant increase.