Abstract: The main S-N curve method is a new method for fatigue calculation, which is widely used in fatigue analysis of welded structures. In order to predict the fatigue life of welded structures under the test load, firstly, the bench model is taken as the boundary condition to make the test load become the input of simulation analysis, and the more accurate dynamic response of weld is obtained through steady-state dynamic calculation. Second, in the master S-N curve method of quasi static calculation process, based on the introduction of dynamic structural stress based on modal structural stress superposition calculation method, the method to obtain the modal coordinates and calculated by the dynamic welding structure modal stress superposition structure, structure dynamic stress and the equivalent stress calculation, the method adopts the Lord S-N curve forecasting life assessment. Finally, the fatigue evaluation software of welded structure modal structure stress method is developed. The software is used to carry out the fatigue evaluation and fatigue test comparison. The results show that: this method can effectively identify the fatigue failure parts of the car body under dynamic loading, which verifies the effectiveness and superiority of this method in fatigue evaluation of welded structures under dynamic loading, and provides a technical basis for the study of fatigue life evaluation of welded structures and the expansion of the main S-N curve method.