Friction stir welding(FSW), a new solid-state joining process, is wildly used in the fields of aviation, aerospace and other high technology industries for many advantages over traditional fusion welding. Computer modeling is an important tool for the prediction and optimization of the FSW process. According to the characteristics of FSW of 7075 aluminum alloy plate, a 3D coupled thermo-mechanical FE model of this process was built under the ABAQUS/explicit environment based on the solution of several key techniques, such as heat boundary condition treating, material properties definition, ALE adaptive meshing technology, etc., and validated experimentally. Then, simulation and analysis of the complex plastic deformation behavior of the process were carried out. The results show that in the stable stage of the welding, the zone of equivalent plastic deformation decreases from the top surface of weld to the bottom surface; the plastic deformation of metal ahead of the welding tool is larger than that behind the welding tool; moreover the zone of equivalent plastic deformation is concentrated behind the tool.