The gas gun is well applied in the ballistic environment simulation of weapons by its excellent performance. In the paper, the launching environment parameters are analyzed firstly, and the characteristic value is found. And then the internal ballistic model of the gas gun is established by applying the similar theory of gas kinetics. Finally, the model is simulated on the computer and analyzed. The conclusions in the paper afford theoretical foundations for the following construction design of the gas gun, choosing of the according equipments and adjusting of the parameters of the gas gun.