The utility model discloses an underground cable partial discharge dual-core online monitoring device which comprises a high-frequency current sensor, a high-frequency signal cable and a monitoring host. The high-frequency current sensor is in communication connection with he monitoring host through the high-frequency signal cable, the monitoring host comprises an insulation shell, a circuit board is arranged in the insulation shell, and the circuit board is provided with a signal amplification circuit, a filter, a high-speed data collecting circuit and a processor. The device is characterized in that the bottom of the insulation shell is hinged to an interface protection box, a pair of butt-joint BNC interfaces is arranged in the interface protection box, one BNC interface is connected with the circuit board, the other BNC interface is connected with the output end of the high-frequency signal cable, the high-frequency signal cable penetrates out of a wire outlet hole in the side wall of the interface protection box, and the input end of the high-frequency signal cable is connected with the high-frequency current sensor through the BNC interfaces. The device is convenient to maintain, and stable reliability of the device is improved.