Using the method of variable specific heat and in consideration of the volume effects of the internal parts, a dynamic model for turbofan engine is established. The numerical simulation for the dynamic process of turbofan engine is calculated as the gas pumping coefficient for power pumping or turbo cooling changes, and the results indicate that the gas pumping coefficient affects the working of aero engine greatly, when the coefficient rises, the stall margin of the LP compressor decreases and that of the HP compressor increases. So different coefficients affect the turbo engine differently.