Abstract: By using general finite element method (FEM) software ANSYS, the three-dimension finite element model of large complicated structure was founded by adopting equivalent simple measures of combinatorial bead and different kinds of elements.It overcomed great calculation and difficult convergence of the thermal elastic-plastic FEM by adopting a series measures which did not influence the calculation precision.At last, using thermal elastic-plastic FEM, the welding distortion of large complicated structure with multipass was successfully obtained.The results showed that by adding knighthead to the place of less stiffness, the welding distortion can be decreased effectively, which provided a good theory basis to control welding distortion.