This paper presents a fiber optic iner tial vibration sensor based on Moiré fringe which is generated by the relative movement of a pair of parallel gratings.The Moiré fringe containing vibration information is transmitted to the signal processing system board by four channel s of optical fiber.The relationship between the Moiré fringe and the vibration signal is dis cussed in detail.By using the signal processing system board,together with the algorithm of subdivision and directio n recognition,the Moiré fringes are converted into the direction and amplitude of the relative displacement.Fu rthermore,an amplitude-frequency characteristic compensation method for broadening the dynamic bandwidth with fla t frequ ency response is developed.Experimental results show that the resonant frequency is 5.35Hz a nd decreases to 0.1Hz after compensation.Frequency response ranges from 0.1Hz to 1000Hz with fluctuation less than 0.011mm.