A conventional axial turbine drived by a pulse detonation combustor heavily challenges the turbine cooling and hot gas sealing. In order to fully understand the physical behavior of ingress and egress effect with the pulse inlet mainstream, a study is carried out to investigate the unsteady flow field and sealing efficiency inside the cavity using the method of unsteady, three-dimensional Computational Fluid Dynamic simulation. The pulse detonation inflow boundary condition simplified using simple exponential decay formulas are applied to the inlet of mainstream passage. The results reveal that the magnitude of sealing gas pressure does affect the pressure and sealing efficiency distribution inside cavity. The sealing efficiency inside the cavity goes through three sub-stages, respectively, "the decline stage", "the plateau stage" and "the recovery stage". when the sealing gas pressure increases, the sealing efficiency of these three sub-stages will increase, and the duration of "the plateau stage" and "the recovery stage" will decrease. As a result, the ability of turbine cavity that resist the ingress of pulse detonation inflow can be augmented with the sealing gas pressure increases.