In order to solve the shortcomings of the existing demodulation system, which can not guarantee the cycle integrity of the driving voltage, a wavelength demodulation system based on realtime reference light path and the tunable FP cavity filter is proposed in this paper. The hardware signal conditioning circuit consists of twostage amplifier circuit and secondorder lowpass filter circuit, which is high signal to noise ratio and low cost; The software analysis and processing module consists of highprecision computer program, which is realtime data acquisition, processing and preservation. What’s more, a sampling point compensation algorithm which is proposed, which can make host copputer produce complete periodic voltage under any circumstance. The experimental results show: the maximum measurement error of the demodulation system is 34 pm, the measurement range is 40 nm, and the maximum tuning speed is 100 Hz. This system can satisfy the requirements of engineering practice.