The line spectra noise radiated by cavity flows greatly deteriorates the acoustic stealth of naval ships, whose formation mechanisms are related to the flow and acoustic modal effects, as well as the acoustic-vibration and flow-sound coupling effects. In this paper, the transient flow, equivalent sound source and acoustic fields of the simple square and typical cavity flows were numerically simulated, based on the CFD/CHA hybrid approaches. The applicability of the numerical methods was verified by comparison with experimental data of water tunnel test. The characteristics of the flow and acoustic modes of the cavity flows were concluded, particularly the effects of the acoustic-vibration coupling of elastic cavity walls and the complexity of cavity inner shapes upon the acoustic mode frequencies were quantitatively calculated. The important regularity, which the line spectrum induced by the effect of the first acoustic mode is the "decisive line spectrum" in the far-field radiated noise spectrum, is summed up. The tendency of the acoustic mode frequencies to shifting sharply towards lower frequencies under the actual cavity conditions was analyzed, indicating the necessity of avoiding the flow-sound coupling. The necessary condition of the cross-sectional area ratio for the related acoustic experiments in water tunnels was quantitatively established through an analytical solution. The research has an important guiding value for the designs of experimental modals.