Abstract: According to at the problems of the lack of safety and reliable insulation on- line monitoring system of mine main ventilator, on the basis of analyzing the development status of monitoring technology, the hardware system for monitoring insulation condition of mine main ventilator high- voltage motor had been designed an d the applicable software of insulation condition on- line monitoring system had been also developed based on C8051F020 microcontroller. The insulation on- line monit oring system was stronger functions to collect, storage and process the signals effectively, such as the insulation resistance, temperature, partial discharge, power freq uency voltage, and then the CPU of monitoring system could transfer the data processed to the host computer so as to evaluate the insulation condition. The monitoring system had been debugged in laboratory and applied to the mine main ventilator in Sihe No. 2 Coal Mine. The results showed that the system function was perfect and run stable, with the monitoring data being highly accuracy and real- time. The system could satisfy the requirement for the on- line monitoring of the high- voltage motor insulation status in mine main ventilator.