The closed loop passive work-points control method is introduced for mixing frequency modulation of light source. The phase-shift signal is picked up from single interferometer signal by directly high frequency modulating light source, and compensated by the low frequency control signal with directly modulating light source. The problem of work-point tracking is resolved by the cooperation of high and low frequency modulation, the system is always working in the most sensitive linear region by way of passive control. The modulation characters of laser source are tested under different frequencies, and the variation tendency of work-point under open-closed loop is compared. Demodulation results of closed loop passive work-points control method and phase carrier demodulation algorithm show that the demodulation relevancy of two methods is above 99%, which proves the feasibility of the proposed method.