In order to solve the problem that the traditional fault indicator in distribution network can’t distinguish and locate the single-phase grounding fault in small current system, a model identification method based on transient zero sequence network is introduced, and a new on-line monitoring device for distribution network is developed. The three-phase probes obtain energy from the line to provide power for their own use through its self power module, use Roche coil to acquire the circuit, and measure voltage by the non-contact sensors, and realize the recording of fault waveform through distributed synchronous recording method based on short distance wireless communication. The communication terminal summarizes the fault waveforms and makes zero sequence vector synthesis. It automatically completes the identification and alarm instruction for single-phase grounding fault, and provides a new solution to solve the problem of the single-phase grounding fault location and improve the coverage rate of distribution network automation. The field experiment results show the practicability of the device and the accuracy of the detection and location of the single phase grounding fault in small current system. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 52020116000F) and Science and Technology Project of Guangdong Province (No. 2015B010128006).