Abstract: The necessity and feasibility of the virtual fatigue test technology of welded structure were discussed firstly. Then a basic theory of a new method called the master S-N curve method,which can be used for assessment of fatigue life of welded structure and published by the ASME (2007) standard, has been discussed as well. Following this discussion,a conclusion has been given that is the new method is resulted from the welded structure fatigue failure mechanism instead of fatigue failure test data. So this method is more suitable as the core algorithm in welding virtual fatigue test then nominal stress method. Finally,two applications have been given,and the results show that during the design phase,the welded structure virtual fatigue test technology based on the master S-N curve method,can effectively identify stress concentrations happed at each weld in a complex welding structure. In fact,this just is needed in design process.