Based on the phase delay function of a quarter-wave plate and combined with a polarizer and a Faraday rotator, a nonreciprocal phase modulator is proposed for the Sagnac loop, which integrates the functions of polarization and phase delay. The proposed structure fundamentally eliminates the influence of delay coil on system sensitivity, and reduces the complexity and volume of the whole system. The measured relative insertion loss of this phase modulator is 3.6 dB, and the relative phase offset angle is 89.9605°. The experimental results are well consistent with the theoretical analysis results. A variable ratio error self-compensation technology is proposed. In the range of -40~40 ℃, according to the different fabrication process, the minimum variation ratio error of the system is no more than ±0.02% when the temperature changes.