This study presents a fiber Bragg grating pressure sensor with a variable cross-section cantilever structure, which meets the needs of surrounding rock-stress measurements in the complex environment of a coal mine. The performances of the designed sensor and a sensor with a constant cross-section cantilever structure are compared, and the relationship between the pressure and fiber grating drift is derived. The cases affecting the sensitivity of different cross-section cantilever beams are analyzed using the finite element method, and the size of the cantilever beam is determined. The effects of different structures and material parameters on the sensitivities of the two cantilever structures are determined through a numerical and finite element analysis. The results are compared with the experimental results of a similar structure. The results showed that the variable cross-section beam should be designed with a relatively thin fixed end and a relatively thick free end. In the final variable cross-section cantilever beam, the thicknesses of the fixed and free ends were 0.5 mm and 2.0 mm, respectively. The sensitivity of the sensor reached 893.45 pm/MPa, which is 294 times that of the bare fiber grating. Even after changing the structure and material parameters, the sensitivity of the variable cross-section cantilever beam was greater than that of the constant cross-section cantilever beam. The sensor structure can meet the needs of coal mine roadway monitoring and exhibits good linearity in the operating range.