Based on in-fiber Bragg grating sensing model, a high-sensitivity in-fiber Bragg grating pressure sensor was proposed by using uniform strength beam and bellow. The analytical formulae between pressure and relative shift of Bragg wavelength of in-fiber Bragg grating were derived. The pressure sensitivity coefficients were also obtained by analytical formulae and experiment respectively. The theoretical and experimental pressure sensitivity coefficients could reach 1.476×10-2 MPa-1 and 1.35×10-2 MPa-1, which were 7455 and 6808 times of that of the bare-fiber grating respectively. The experimental result fits the theoretical one well. At the same time, it was pointed out that the pressure sensitivity coefficients of this kind of sensor could be very large by regulating the parameters of strength beam and bellow till the in-fiber Bragg gratings were damaged.