The invention relates to an insulation detecting system for power transmission cables. The insulation detecting system comprises an electric power cable. A GIS (geographic information system) terminal is connected with the electric power cable, a grounding circuit of the electric power cable is provided with a current sensor, an amplifier is connected with the current sensor, a signal filter is connected with the amplifier, an A/D (analog/digital) converter is connected with the signal filter, a computer is connected with the A/D converter, a data concentration collector is connected with the computer, an environmental temperature collector and a resistance information collector are connected with the data concentration collector, a temperature sensor is connected with the environmental temperature collector, insulation resistance testers are connected with the resistance information collector, GPS (global positioning system) positioning devices are connected with the insulation resistance testers, and the electric power cable is connected with the insulation resistance testers. The insulation detecting system has the advantages of wide application range, accuracy in fault detecting and fault positioning, reliability in running, low power consumption, long service life and convenience in debugging and operation.