Abstract: Hot spot stress method used for evaluating fatigue strength of welded structure has obtained more and more application,the FEM is vital in this method.The key factors influencing calculation precision are extrapolation methods and mesh size.The FEM analysis has been done in three points bending welded joint,the hot spot stresses of different extrapolation methods and mesh size are compared.The results show that mesh size has greatly effect on hot spot stress,and element type should be considered in meshing,however,extrapolation methods have little effect on calculation precision relatively,two points linear extrapolation can be applied to welded joints in general.