Abstract: A new approach combining fluid network and unsteady simulation method was developed to predict the unsteady characteristic of pressure,temperature and mass flow rate of the flow in engine.The air cooling system was simplified into a fluid network comprised of nodes and branches.Conservation equations were solved by a newly introduced method to deal with the computational unstabilization problem.As a validation,the program was applied to a model which had provided a correlation,and the computational results show good agreement with the benchmark.Then the program was used to predict the unsteady characteristic of flow in the cavities along the axis in the engine.