The design of a blockage adjustor at the exit is commonly used in simulating the impacts of backward pressure on the isolator flow field. Numerical simulation was adopted to study the effects of different block modes. Three modes were compared, including installing a flat on the ramp side wall, a flat on the cowl side wall and a wedge in the middle of the flow field. The results show that due to the asymmetric boundary layer on the ramp wall and cowl wall, the shock trains formed by them are all next to the cowl side. The intensity of the shock train generated by the second mode is stronger and the boundary layer separates more evidently, which chocks the isolator more easily and poses difficulty for the study of shock train in experiment. The other two modes can generate similar flow field, but the first one can be realized more easily. 