To reduce the launching recoil of firearms, a numerical calculation model of internal ballistics based on combustible cartridge and recoilless was established. The working principle and launch process of the rear jet gas recoilless launch system are analyzed, and the zero-dimensional interior ballistic model during the launch process is established, to simulate the performance of ballistic system. A recoilless launch trajectory system verification scheme was designed, and internal ballistic was conducted. The numerical simulation results were compared with the experimental results. The results indicate that the model can accurately simulate the changes in chamber pressure during the launch process.