Abstract: Numerical simulation of the flow field of multi-cycle pulse detonation engine (PDE) is helpful for understanding the engine principle and performance analysis, and can provide the useful information for PDE design. The reactive Euler equations and a 9-species, 20-elementary-reactions model for hydrogen-air combustion were used to simulate the flow field of the first six cycles for a simplified two-dimensional PDE with a convergent-divergent nozzle. Results show that there are many distinctions between the single cycle and final steady flow fields. It needs five cycles until the flow fields reach the steady state, and the nozzle plays an important role in the engine internal flow.