A detailed experimental research on characteristics of the hydrodynamic pressure spreading in the base gap of the arch inverted plunge pool was carried out. The features of the distributon of its ampitudes and spectral density and its characteristics in time domain for two conditions, I.e., (1) a through gap and (2) a blind gap were given. The influence of the damp at the joint opening upon the hydrodynamic pressure inside the gap was also investigated. Accordingly, the stabilization mechanism of the arch invert plunge pool was worked out.