The flow field of plastic deformation and the temperature field during the process of friction stir welding can directly affect the structure evolution of the welded joint, and also have a great influence on mechanical properties of the final joint. In this respect, therfore, a lot of researches have been carried out. The recent research results show that this process is an extremely complex coupled thermal-mechanical process, the temperature field couples together with the flow field of plastic deformation of the material. In this paper, the coupled process is simulated based on hydromechanics and heat transfer theory; the flow field of plastic deformation and the temperature field are calculated; an experiment is designed to measure the temperature field, which indicates that the computed results can exactly describe the coupled thermal-mechanical condition of the friction stir welding in quasi-steady-state.