The process of underwater vehicles exiting the barrel is often accompanied by complex flow of high-temperature and high-speed gas inside the barrel and multiphase mixed flow of water and gas at the barrel mouth， presenting complex flow states and changing patterns，which have important impacts on the launch power，load，and motion attitude of the vehicle after exiting the barrel. This article takes the launch of a certain underwater vehicle as an example and uses CFD method to conduct numerical simulation research on the launch of a high-temperature gas propulsion underwater vehicle. By comparing the horizontal and vertical displacement under different grid densities，the convergence of the numerical simulation in this article is demonstrated. Using a validated numerical model，the process of high-temperature gas propulsion for underwater vehicles exiting the barrel is simulated under different initial launch angles and lateral inflow effects. The deflection angle and velocity of the vehicle after exiting the barrel are obtained，and the typical characteristics of the velocity and temperature fields during the launch process are clarified，providing theoretical guidance for design of underwater thermal launch system.