Based on the effect of fiber Bragg grating (FBG) pressure difference sensitivity enhancement by encapsulating the FBG with uniform strength beam and metal bellows, a FBG pressure difference sensor is proposed, and its mechanism is also discussed. The relationship between Bragg wavelength and the pressure difference is derived, and the expression of the pressure difference sensitivity coefficient is also given. It is indicated that there is good linear relation between the Bragg wavelength shift and the pressure difference of the sensor. The theoretical and experimental pressure difference sensitivity coefficients are 38.67 and 37.6 nm/MPa, which are 12890 and 12533 times of that of the bare FBG, respectively. The pressure difference sensitivity and dynamic range can be easily changed by changing the size, Young's modulus, and Poisson's ratio of the beam and the bellows.