This work deals with the welding simulation by the Finite Element Method (FEM). For the tube plate welding structure features of Membrane water-wall, the numerical computation method is applied to do the real-time simulation research on the temperature field and stress field during the welding process. The implemented models include a moving heat source, temperature dependence of thermo-physical properties, elasto-plasticity, non-steady state heat transfer, and mechanical analysis. The thermal problem is assumed to be uncoupled from the mechanical one, so the thermal analysis is performed separately and previously to the mechanical analysis at each time step. The mechanical problem is based on the thermal history. A special treatment is performed on mechanical elements during the liquid/solid and solid/liquid phase changes to account for stress states. The established simulation analysis method of welding temperature and stress field can provide theory foundation and direction for optimizing the welding structure design and standardizing the welding parameter.