Based on the hot compression tests of AZ31B magnesium alloy, the trend of flow stress was analyzed and constitutive equation was established. Using Deform-3D Microsoft, the process of magnesium alloy wheel was numerical simulated. The effect of fraction coefficient and heating temperature on extrusion load and radial load on die surface was studied. It was shown that variety of these loads was tardo at prophase back-extrusion while violent at anaphase stage. The fraction coefficient was an important factor in total compression load and radial load on die surface was affected obviously by forming temperature.