The computation fluid dynamics software is used to simulate injection pipe with same nozzle radius,and the airflow distribution characteristics of different inlet mass flow rate are obtained. Then the unsteady flow field model of the injection pipe is established,and compared with the steady- state simulation results,the steady and non steady state simulation results error is less than 1%. It is feasible that the unsteady flow field in the nozzle can be simplified as steady flow field. Along the flow direct...