Abstract: In the shear layer of the open cavity flow, the vortex interacts with the pre-transmission sound wave, causing self-sustaining oscillation. For a cavity model with a length-to-depth ratio of 7, the dynamic characteristics of the shear layer in the cavity were tested under the incoming flow condition of Mach number 0.9 by the pulsating pressure measurement technology, and the propagation law of the modal noise in the shear layer is revealed by the spectrum analysis and cross-correlation analysis. The results show that the superposition of the monotonically increasing broadband noise and cosine-like modal noise in the shear layer causes the wave-rise characteristics of the overall dynamic of the shear layer. The modal noise propagates in the reverse flow direction, its velocity is also cosine-like, and the change trend is consistent with the modal noise amplitude. Combined with the Rossiter mode estimation theory, it is revealed that the interaction between modal sound waves and vortices of the same frequency produces a standing wave-like phenomenon, resulting in periodic changes in the power spectrum density and propagation velocity of the modal noise along the flow direction.