A wavelength demodulation method for the active fiber grating sensor is proposed. A tunable filter is used for wavelength scanning, and an etalon is used for wavelength calibration in this method. The corresponding demodulation system is set up. The feasibility of the demodulation method and the accuracy and stability of the demodulation system are verified experimentally. The results show that the proposed method can measure the wavelength of the active fiber grating accurately. When the wavelength range is from 1533 nm to 1550 nm, the accuracy of the demodulation system is higher than 6 pm, and the fluctuation range is within ±5.7 pm.