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At present, the rail transit system adopts DC electric traction. The current required by the vehicles flows out from the traction substation, and then pass through the OCS (overhead contact system) and the running rails. At last, the traction current will return to the traction substation. A proportion of the traction current will leak to earth, such leakage current is called stray current. Generally, the stray current can be calculated indirectly by using the following parameters: the running rail longitudinal resistance, running rail-to-earth resistance and running rail-to-earth voltage. It is necessary to develop on-line monitoring device of stray current in metro. In this paper a novel stray current on-line monitoring device based on embedded microcomputer system is developed. According to the new method presented in this paper, the on-line monitoring device will be installed along the rail, and connected each other with communication cable to form a real-time monitoring system. Comparing with the traditional method that installing the monitoring device at the equal distance along the rail, this novel method could not only reduce the system complexity and funds budget, but also enhance the monitoring accuracy and precision.