Based on the vehicle virtual prototype simulation, a new method for auto parts fatigue analysis is presented to avoid the real vehicle tests. In consideration of the modal stress recovery (MSR), the method can accurately solve the flexible multi-body system dynamics under the modal coordinate. By using MSR, the loading spectra of auto parts can be acquired after the virtual prototype's road tests. Then the fatigue test of the parts can be conducted with the loading spectra. This test method was applied to a lower control arm (LCA) of a car's front independent suspension. The result indicates that this virtual fatigue test method is an effective means for auto design and test.