Nuclear Magnetic Resonance Atomic Gyroscope has attracted an extensive attention due to its high precision, compact size and less cost at home and abroad. Among numerous techniques for obtaining the detection target, the balanced polarimetry technique is a more general method which monitors the rotation of the polarization plane of the probe beam. Due to miniaturization installation, the size effect is obvious. So there is an inevitable azimuth deviation between optical elements that leads to a bias of the weak optical signal, which extremely restricts the extraction and amplification. For the purpose of improving the detection performance, the differential detection principle of the laser polarization was analysed and a circuit method was proposed which could automatic eliminate the photodiode current bias based on the proportional-integral (P-I) feedback loop. A low-noise and high-gain preamplifier for silicon photodiode was presented. Finally, combined with the NMRG prototype system, the simulation analyses and experimental results were given, which verified the validity of this method.