Abstract: Based on the principle of two-stage light gas gun, an interior ballistics numerical analysis program was developed to analyze the performance and operation of two-stage light gas gun. The included numerical models were: a quasi-one-dimensional, two-phase hydrodynamics model to simulate the combustion of solid propellant in the gunpowder chamber and pump tube, an unsteady, quasi-one-dimensional, compressible flow model to analyze the flow of hydrogen gas between the piston and projectile, a one-dimensional, ideal viscoplastic, extrusion model to simulate the motion of piston in high pressure section, and a friction model combined the classical law of frication to describe the motions of piston and sabot in the pump and launch tubes. The friction and heat transfer to the tube wall for gases and solid media are also modeled. The governing equations of gas flow were solved by CFD methods, which have second-order accurate in space and time. The interior ballistics characteristics of two-stage light gas guns of CARDC were analyzed under the typical test operating conditions, and the numerical projectile velocities were agreed well with that of test results. With the help of this program, the motion process of piston and the history of projectile acceleration loads could be obtained. The proposed method provides a way to optimize the operating conditions and improve the performance of two-stage light gas gun.