A novel demodulation method of fiber Bragg grating sensor is presented. Based on the filter characteristics of long-period grating, it is used as the filter component of the demodulation system. An experimental system is built up to achieve one or multi channels of sensor demodulation. Using this method based on intensity, it can achieve linear demodulation with a bandwidth of 5 nm. And the resolving wavelength is 0.002 nm, with the demodulation speed up to 10 kHz. The experimental results were analyzed in detail. The advantages of this system are an all-fiber design, quasistatic and dynamic operation, and potential high-speed demodulation.