The medium and low voltage distribution network has a large number of faults with complex types.The low level of fault monitoring makes it impossible to discover and clear the faults in time after a fault occurs. In response to his problem, this paper analyzes the operating conditions of 10 kV distribution transformers at different load rates, winding connection groups,threephase unbalance rates,and system voltages.The changing law of electrical signal on the lowvoltage side is obtained for typical faults such as the disconnection of medium and low voltage lines, singlephase loss of distribution transformers, and singlephase grounding of low voltage line.Then relevant fault diagnosis rules are summarized and formulated to provide a basis for the judgment of medium and low voltage faults.On this basis, a comprehensive fault judgment platform including the data integration, fault warning, fault research and other functions is built and successfully applied in the actual operation and maintenance, repair and decisionmaking in Jiangxi power grid. The results show that the platform can detect faults in low and medium voltage distribution network timely, reduce user power outage time and improve power supply reliability.