Abstract: In order to solve problems of consuming too much time and resource and unstable precision caused by environment of existing off-line detecting method for dielectric loss of high-voltage cable of coal mine underground,a micro-current sensor was designed for measuring micro grounding current of cable's earth line,and by use of the sensor,an on-line detecting system for dielectric loss of mine-used high-voltage cable was designed based on zero-crossing point comparing method.The composition and on-line detecting principle of the system were introduced.The results of simulation and test showed that the system has stable performance,high measuring precision and strong anti-interference ability,and can on-line measure dielectric loss of high-voltage cable accurately.