Provided is a power distribution network cable insulation detection system. The power distribution network cable insulation detection system comprises a background computer system installed in a master control center and a low-current grounding line selection insulation monitoring device installed in a substation and a communication network which connects the background computer system and the low-current grounding line selection insulation monitoring device. The low-current grounding line selection insulation monitoring device is responsible for identifying and detecting high-frequency capacitance and current signals released when transient grounding fault happens to cable outlets in the substation, recording data and forming an oscillographic document, then transmitting information of the oscillographic document to the background monitoring computer system through Ethernet communication network for analysis and statistics, and therefore the data analysis on insulation state of power distribution network is achieved. Weekly statistics, monthly statistics and annual statistics of insulation monitoring reports, as well as Specific Operating Requirement (SOE) reports are generated automatically. The power distribution network cable insulation detection system can effectively screen faults on electric insulation of the power distribution network system in an early deterioration stage, give expert suggestions on wire repair, and therefore prevent faults before faults happen and effectively improve operational safety of the power distribution network system.