Abstract: On the basis of true flow stress-true strain curves of AZ31 magnesium alloys obtained from hot compression process, the corresponding flow stress equation was presented and the microstructure of the compressed alloy was analyzed. The square tube extrusion process for AZ31 magnesium alloy was simulated by using the HyperXtrude software. The results show that the flow stress decreased with the increasing temperature and could reach the steady state quickly above 350 ℃, which is suitable for processing forming. The twins in hot compression specimens reduced with the temperature increasing, moreover the grains were growing up. With the high strain rate, the fine grains similar to “necklace” appeared on the grain boundaries due to the insufficiency of dynamic recrystallization. The finite element simulation results show that the metal velocity in the corner of the square tube was lower than that in the center, but could distribute well as the optimization of the bearing part in the HyperStudy. The die designed on the optimized die bearing model proved to be capable of producing the qualified products in practice.