This paper proposes a gas generator-based numerical calculation method for the dynamics of tilted launching systems. The fractional motion model of gas ejection with fixed tilted angle is constructed, which reveals the internal trajectory of high and low pressure chamber. With the theoretical model, a fast calculation method for internal trajectory parameters, such as the pressure of high pressure chamber, process time, velocity and overload, etc. It is meaningful for the design and fast evaluation of ejection device. Meanwhile, a testing method for the dynamics is proposed. It can achieve the measurement of the performance parameters of gas generator and the dynamics data using few measuring modules, and has low cost, simple operation, and easy testing. Finally the effectiveness and correctness of the numerical and testing method are verified through experiments.