On the basis of the analysis and comparison of the existing power cable insulation monitoring methods，it designs the online
monitoring system of the underground power cable insulation approach for the operational characteristics of the underground coalmine cable
lines． The system uses the low-frequency signal injection method，injecting the low-frequency signal into the cable through the reactor
neutral point． It constitutes a loop of the low － frequency signal，the cable insulation resistance and the Earth． Under the circumstances
of injecting certain low-frequency voltage，it can achieve on － line monitoring of the cable insulation，by detecting the value of
the low-frequency current-line，so the problems of poor reliability and low sensitivity on-line insulation monitoring of the power cable are
solved better． The experiment shows that this method has a low error and high accuracy in detection; it can monitor not only the single
cable insulation but also the multiple ones． It can apply to both the main cable circuits and the branch ones．