From the mechanical point of view, welding processes can be modeled as a coupling problem between heat conduction and thermal elastic–plastic problems. Such a welding mechanics problem generally requires a large amount of computational time due to its nonlinearity as well as a lot of time steps with a moving heat source. To overcome this difficulty, we are developing a large-scale welding simulator based on the domain decomposition method, which is one of parallel finite element methods. In the present paper, the methodology of the domain decomposition method that is applied to welding analysis is presented, followed by the algorithm. Then, a bead-on-plate problem, which is a popular benchmark problem in the field of computational welding mechanics, was analyzed by the simulator. The bead-on-plate problem was successfully analyzed within very small numbers of iteration steps of the Newton–Raphson and conjugate gradient methods.