The invention discloses an AC-DC system fault detector and an operation method thereof, and belongs to the field of power monitoring equipment. The detector comprises a detector shell, a device powermodule, a signal extraction module and a processor chip module are arranged in the detector shell, the device power module is used for supplying power to the detector by a voltage stabilization circuit, the signal extraction module is used for detecting a voltage signal, the processor chip module is used for processing voltage signal data by a data processing loop, the signal extraction module comprises a data filtering loop and a data amplification loop, a core component of the data filtering loop is a lowpass filter, a core component of the data amplification loop is an operational amplifier, a core of the data processing loop is a MCU, and the detection of the detector is on the basis of a DC blocking capacitor detection technology. By the detector, the fault removal time of an AC-DC system is reduced, the running safety of a substation during fault removal is improved, the detector is convenient to carry and operate, the real-time state of the DC system is observed by the detector,and the detector is simple and clear and is high in practicability and wide in application range.