The invention relates to a capacitor partial-discharging ultrasonic gating device. The capacitor partial-discharging ultrasonic gating device consists of an insulation shell, a circuit board which is arranged in the insulation shell and an insertion port which is arranged outside the insulation shell, wherein the insulation port comprises an electrical signal input port, a sound signal input port and a partial-discharging signal output port, the electrical signal input port is externally connected with a mutual inductor, the sound signal input port is externally connected with an ultrasonic sensor, and the partial-discharging signal output port is externally connected with an electrical signal collector. In a strong electromagnetic environment, a partial discharging signal can be separated from an interference signal by utilizing the strong interference resistance capacity of the ultrasonic through the ultrasonic gating, and the partial discharging signal is isolated and amplified. By adopting the capacitor partial-discharging ultrasonic gating device, the internal partial discharging signal of the electric power capacitor can be effectively recognized, and the insulation level of the electric power capacitor can be accurately evaluated.