In this Letter, the liquid crystal variable phase retarder is applied for the accurate modulation of the laser power in a detection system and the construction of a system that suppresses the influence of laser noise on the gyro’s bias instability. A closed-loop control method for a laser noise suppression system is proposed. We obtain a power stability index of 0.038% in a 3-h continuous test, and the nuclear magnetic resonance gyroscope bias instability reaches 1°/h. The proposed control method effectively improves the signal-to-noise ratio of the gyroscope detection signal, which lays the technical foundation for future research work.