Based on the principle of reflective electro-optic phase modulation combined with polarizer and Faraday rotator, a non-reciprocal phase modulation scheme for fiber ring interferometer is proposed. The structure makes use of the different tangential electro-optic characteristics of lithium niobate crystal to form two interference signals with adjustable phase on the ring interferometer, especially by introducing a total reflection prism whose energy loss is small, so as to realize non-reciprocal phase modulation, the half-wave voltage of the phase modulator is reduced effectively. The measured reflection (modulate twice) structure phase modulation half-wave voltage is half lower than transmission (modulate once). and used in the debugging of fiber-optic ring interferometer to obtain the linear correlation between the detected signal and modulation signal phase. It provides a feasible reference for the development of the next principle prototype.