Four techniques of dynamic mesh are Spring-based Smoothing, Dynamic Layering, Local Remeshing and the ALE method in the CFD transient simulation of engine. Based on those techniques, The authors analyze the improved measure of technique of dynamic mesh in the CFD transient analysis of the engine and propose the combined technique of dynamic mesh based on the finite volume method which is appropriate for. The CFD transient simulation of engine. At last, the 2D dynamic mesh method is applied to the typical pent-roof engine and the paper simulates the transient 3-D flow field at the tangent inlet of engine by applying the combined technique of dynamic mesh based on the finite volume method. The result of the simulation agrees well with experiment.