Abstract: By means of macroscopic and microscopic analysis, energy spectrum analysis, density test and hardness test, the anomaly-defect in the macrostructure specimen of AZ40M magnesium alloy forging was analyzed. The results show that the anomaly-defect was segregation of compounds composed of elements of Al, Mn and Fe, and it belonged to regional segregation. The segregation had influence on the local density and hardness uniformity of the material. At the same time, due to the large amount of Fe element in the region, the corrosion resistance was also greatly reduced.