Microstructure observation by optical microscope and numerical simulation by MSC software were performed to study the reason of ring crack at the bottom of the extruded magnesium alloy tube. On this basis, the effective protective measures were proposed by numerical simulation analysis. The result shows that: there is a significant difference between the direction of backward extrusion metal flow and the direction of the original metal flow direction. Therefore, the plastic deformation is difficult, and then the crack initiates owing to severe stress concentration. Through simulation analysis, it is found that both increasing the bottom thickness and changing the bottom position could effectively prevent the crack.