Abstract: Based on the coupled equation of classical interior ballistic theory and computational fluid dynamics theory, the gas flow in bore of piston high-low pressure launcher is modeled using Fluent and analyses the gas flow in the launch process to optimize the structure of piston high-low pressure launcher in this paper. Experiments are carried on to validate the analyses. The results of simulation and experiments show the details of interior flow field according to its 3-D model, which provides the theoretical basis for the design of piston high-low pressure launcher.