Based on the fatigue test results of SAE 1045 medium carbon steel, the multi-axial fatigue life predication models for different ratios of torsional nominal stress amplitude to the axial one are studied respectively. Then a unified multi-axial fatigue life predication model is founded. It is found that the different ratios induce different models. In order to get the relationship between the axial model and the unified multi-axial model, it is supposed that the fatigue strength exponent and the fatigue ductility exponent are constants, while the fatigue strength coefficient times 1.5435 and the fatigue ductility coefficient times 0.6713, the maximum principle stain amplitude replaces the axial push-pull strain amplitude, then the revised axial fatigue life predication model is equivalent to the unified multi-axial model. Therefore, the revised axial fatigue life predication model can be used to predict the multi-axial fatigue life for any ratio of torsional nominal stress amplitude to the axial one.