<p>The new generation launch vehicle fuel tank is made of 2195 Al-Li alloy and is welded by friction stir welding. When welding the key structure, the temperature of the bottom surface of the weldment will induce thermal deformation or even melting of the internal material, which will reduce its service performance. The temperature distribution in the welding area affects the plastic flow of the material and the evolution of the microstructure of the joint, which in turn affects the mechanical properties of the joint. In the existing simulations, the tool geometry is often simplified, and the nonlinear variation of the friction coefficient with temperature is often ignored. However, the complex geometric features of the tool significantly influence heat generation and material plastic flow, and the friction coefficient directly influences the state and friction behavior of the material at different temperatures. To overcome the limitation of the difficulty in measuring the weldment’s bottom temperature through experimental methods and characterize the temperature distribution throughout the FSW process at low cost and high efficiency, a temperature field simulation model during FSW of 2195 Al-Li alloy, which considered the tool’s complex geometry and the temperature-dependent friction coefficient, was established and validated through experiments. It reveals the law of temperature distribution. The influence of welding speed and rotational speed on the temperature field was explored. It provides a reference for the selection of process parameters. This research lays the foundation for achieving high-quality welding of tanks and temperature control of the bottom surface.</p>