Abstract: To investigate the dynamic response of the support arms of hydraulic arc steel gates under impact loads during normal operation conditions and to predict possible failure modes, an arc steel gate from a certain hydropower station was selected as the research object. The nonlinear finite element method was used to simulate the process of the support arms of the steel gate being impacted by floating debris under hydrostatic pressure, obtaining and analyzing the impact force, displacement, and overall deformation characteristics of different positions of the arc steel gate support arms. The results show that the gate failure process begins with out-of-plane buckling of the impacted support arms or their diagonal bars, followed by the movement of the gate towards the hinge support on the side close to the unstable support arm, causing the hydrostatic load borne by another support arm to instantaneously increase and be crushed, ultimately leading to the overall instability and failure of the gate. With the increase in the mass and impact energy of the floating debris, the force and displacement experienced by the impact position increase accordingly. With low impact energy, even if the stress at the impacted part of the support arm exceeds the yield stress of the steel, the gate still has a certain degree of stability; the maximum impact energy that different parts of the gate arm can withstand varies significantly, and the position of the impacted arm near the hinge support is prone to cause structural instability and failure of the gate; the higher the head of water above the weir, the easier it is for the gate arm to become unstable when impacted. The research results can provide a reference for the safety assessment when the gate is impacted and for identifying the components of the gate most prone to instability.