Numerical simulations of a single ejector driven by one-and two-pulse detonation engines (PDE) were performed to investigate the thrust augmentation mechanism. The systems of conservative laws of inviscid fluid combined with the one-step chemical reaction model are discretized in Cartesian coordinates using the high resolution hybrid Roe/HLL scheme and adaptive mesh refinement method, and integration in time is performed with a second-order Runge-Kutta method. Our results described the propagation processes of both primary detonation of PDE and secondary induced detonation wave inside the ejector. For a single detonation tube, the detonation gases were expelled from the ejector inlet first, then, a large amount of ambient air is entrained into the ejector with the development and propagation of detonation wave along the ejector. However, for the two-detonation tube case, both the level and duration of expelled flow increase due to the interaction of shocks near the intersection of tubes and ejector, which causes less ambient air to be entrained into the ejector and leads to a decrease of ejector thrust augmentation. Our results agree well with corresponding experimental results.