In the friction stir welding（FSW）of thick plate or high-strength alloy,tools easily rupture under the high stress and high temperature. Therefore,it is of great significance to analyze the tool load bearing capacity. In this paper,a Fluent fluid-solid coupling model is established to simulate the thermal mechanical process for the FSW process of 9.53 mm thick AA6061 aluminum alloy. The resultant temperature and pressure fields of tool pin are imported into ANSYS model for the structural analysis. The effect of welding speed on the maximum shear force and the safety factor is investigated. It is shown that the tool is the most likely to rupture at the root of tool pin. With the increase of welding speed,the maximum shear force of tool increases,resulting in the decrease in safety factor and load bearing capacity. The simulation results of the model are basically consistent with the experimental data.