Abstract: The relay protection based on the current principle is the most widely-used main-protection devices in power systems, its measurement circuit abnormity will directly lead to incorrect tripping or missoperation of protection equipment, which is the high-risk incident that power systems should be vigorously prevented. Polarity is an important characteristic in current measurement circuits of protection secondary systems, but its existing detection methods are still cumbersome and complex, and often bring wrong or leakage inspection events, which will seriously affect the safety and reliability of power systems. Therefore, it is necessary to research more simple and reliable polarity detection measures to ensure the correctness of polarity of current measurement circuits. So this paper defines the generalized ratio, and establishes a polarity diagnosis model of protection current measurement circuits, and analyzes the influence of comprehensive error of measurement circuits to the diagnosis of the polarity faults, and finally obtains the polarity fault criteria of current measurement circuits. Simulation analysis demonstrates the effectiveness of the proposed method.