Abstract: Cavity structure is extensively employed in the aerospace vehicle components and ground vehicles. The complex characteristics of the flow and acoustic fields is one of the key problems that must be considered in the design of the associated practical engineering. In the cavity flow, the hydro-acoustic interaction plays an important role in the self-sustained oscillation. Accurate identification and decomposition of the hydrodynamic and acoustic mode is the key to improving the understanding of the hydro-acoustic interaction and the associated energy transfer mechanism. In this paper, the two-dimensional Navier-Stokes equation is directly solved to conduct numerical simulation of open cavity flow with inflow Mach number of 0.8 to obtain the high-order accuracy unsteady flow field. Adopting Doak’s momentum potential theory, the momentum of the flow is decomposed into three parts of the hydrodynamic vortical component, the hydrodynamic entropic component and the acoustic component. The physical properties and the associated energy transfer characteristics of each component are analyzed. The results show that the hydrodynamic vortical and entropic components exist only in the near field, which are convected downstream with the main flow at the speed of shear layer convection. The spatial distribution of the vortical and entropic components are concentrated in the shear layer and resembles each other. The hydrodynamic energy carried by the vortical component is transported from the inside of the shear layer to the outside of the shear layer and to the rear-end of the cavity while the energy carried by the entropic component is continuously transported to the shear layer and then dissipated there. The acoustic component exists in both the near and far field, and the spatial distribution of the acoustic component exhibits a classical compression-divergence pattern. The acoustic energy is radiated from the rear-end of the cavity and propagates to the upstream and the far field at the speed of sound.