Abstract: A hydraulic expression of negative damping ratio of a tainter gate when a tangential vibration resulting from changing waterflow taking place on such a rigid body is introduced to define the negative damping ratio corresponding to the parametric resonance condition caused by the longitudinal constant force of compressional members and alternating force. Various types of vibration of the tainter gate are simplified into two kinds of compressional members, i.e., two simply supported ends and one simply supported end with one elastically supported end, and the relevant formula of Euler critical load of 1st kind is given. A formula of longitudinal force at member end is thus given approximately through kinetic equation. The reference values of pulsating pressure acting on supporting hinges are also given according to experiments, which will lay a foundation for discussing further the typical case of dynamic instability.