Research method: Combining subway's actual conditions with the deciding target set in standards for the hazards of the stray current, selects polarized potential of the underground metal as the monitor parameter, and utilizes Cu/CuS04 as comparing electrode because of its qualities such as stable voltage, not easy to be polarized, low resistance and mechanical intensity. The core of the system's hardware is the microprocessor based on ARM7 whose high speed and rich connecting sources can easily realize the monitor and control function.