As a flame stabilizer of scramjet engine, the cavity has attracted much attention. The interaction between cavity shear layer and background waves affects the performance of cavity flame stabilizer. To deeply analyze the influence of background shock wave system on the flow in the cavity, a cavity with an aspect ratio of 13.3 is designed. The leading-edge shock wave of the cavity model and the wall interation shock wave on the upper wall of the wind tunnel are used as the background waves. The interaction between the background waves and the cavity shear layer is carried out in a Ma=2 direct-connected wind tunnel. The high-speed schlieren system is used to capture the transient flow field, focusing on the dynamic characteristics of the background waves and the cavity shear layer. The main coherent structures in the flow field are studied by the snapshot proper orthogonal decomposition of the schlieren sequences. The frequency characteristics of the flow field are analyzed by fast Fourier transform and continuous wavelet transform. The results show that with the interaction between the background waves and the shear layer, the shock-wave structure produces large-scale oscillation and the flow structure in the cavity produces small-scale pulsation. With the Fourier transform analysis of the shock wave position, it is found that the dominant frequency of the shock wave oscillation is mainly concentrated in the range of 100Hz-500Hz. With〓the spatial Fourier transform analysis of the schlieren image, it is found that the flow field pulsation below 5 kHz is mainly caused by shock-wave oscillation, and the flow field pulsation above 5 kHz is mainly caused by the flow structure in the cavity.