A coupled thermal–mechanical model based on the Eulerian formulation is developed for the steady-state dissimilar friction stir welding (FSW) process. Multiple phase flow theories are adopted in deriving analytical formulations, which are further implemented into the fluent software for computational fluid dynamics analysis. A shear stress boundary at the tool/workpiece interface yields a much more reasonable material distribution compared with a velocity boundary condition when the involved two materials have quite different physical and mechanical properties. The model can capture the feature of embedded steel strip in aluminum side, as observed in weld cross sections from experiments. For further evaluation, the calculated flow and thermal response are compared with experimental results in three welding conditions, which generally show good agreements.