With the rapid development of high-speed-railway, environment around high voltage device on train roof becomes very complicated. Most train accidents happened due to occurrence of flashover on roof insulator, but the insulation condition estimation of insulator in such environment is much difficult. To ensure the insulation property of electric equipment, and guarantee the operation safety of high-speed-train, here established an instrument with high reliability which can on-line monitor insulation condition of roof insulator and give out advanced alarm before the incipient insulator flashover. The instrument consists of three parts, Data Acquisition & Sensor, Data Processing and Back Processing. Anti-interference and protection methods are processed to Rogowski coil sensor for better leakage current signal. To avoid the fluctuation from railway power supply, four modules are set to filter the power supply waveform. Through laboratory measurement, it is shown that the leakage current and the impedance angle can be detected by the instrument accurately. From the comparison of leakage current and impedance angle results under different moisture condition and the alarm operation when leakage current value reached threshold, this instrument can give out enough information for staff to understand the insulation condition of insulator.