The utility model provides an underground line fault detector, which is also composed of a transmitting device and a receiving device. A receiving and transmitting device is composed of a power source, a low frequency and ultra low frequency oscillation device, and a power amplifier. The receiving device is composed of a power source, a frequency selecting amplifier, a power amplifier, a function selection switch, loudspeaker meter, a probe or an inserting rod. Because of the adopting of a common battery as the power source, adaptability is improved. Main circuits adopt integrated blocks, and volumes and weight are reduced. In addition, the utility model adopts an advanced peripheral circuit and component structure, the performances of the utility model is obviously improved.