The aim of this study is to solve the problem of abnormal vibration during non-load working of No.5 arc gate of the surface outlet of Wanyao Reservoir in Jiangshan City. The causes of abnormal vibration were analyzed firstly by detecting the current position of the gate hinge and embedded plates, the gate position at different openings, and then by prototype tests including vibration test, working stress test, lifting and closing force test, as well as theoretical modal analysis on arc gate vibration. Results showed that there were two main causes of the arc gate abnormal vibration: the first, when the gate opened to 7.6m, the elevation of the right point on the upper main beam was 88mm higher than that of the left point; the second, deviation of the elevation of left hinge axis exceeded the standard tolerance. Abnormal vibration was especially obvious on the left arm, and mainly happened when the gate was opened within 4.39m, which is a rigid rotational vibration around the upper axis. The research is a significant reference for the safety assessment and fault treatment of arch gate.