Abstract: Unsteady flow characteristics and complex flow-field structures are main phenemena as transonic or supersonic speed flow past open cavities. Physical mechanism and rational predicted method of flowoscillations in open cavities remains as a research hotspot for years. Many explanations on the flowoscillation in the open cavities are proposed now and then. The method presented by Rossiter is the most feasible, which indicates a theory model for sound occurrence and feedback in the open cavities. The method recuenes that some vortices are generated from flow separation in shear-layer over the open cavity, then the vortices move to the rear region of the cavity at some velocity, and impingement of the vortex and the rear-wall of the cavity occurs. Some sound waves spread abroad to the cavity fore-wall, which induces some new vortices generation, and the vortices shed, move and impinge on the cavity rear-wall. New sound waves occur. Therefore, this paper presents some analysis of the parameters describing flow-oscillations based on non-dimension analysis method according to predicted method on self-sustained flow-oscillation modes in open cavities. A new selected method on the constant value in Rossiter's and Heller's semi-experience formula is presented. The result indicates that the method is feasible by comparing with wind-tunnel experimental result and overseas references.