We investigated a fiber-optic Fabry-Perot high-pressure sensor for marine applications, and established a finite element method (FEM) numerical model to analyze the full scale cavity length variation of the sensor. The numerical simulation results showed that the full scale cavity length variation in the FEM model was between the value of the clamped model and that of the simply supported model. With the decrease of the cavity radius and the increase of the silicon diaphragm thickness, the length variation of the FEM model deviated from that of the clamped model. We introduced clamped boundary condition deviation degree β to quantify the deviation degree. We produced three types of sensors and carried out pressure experiments. The experimental results showed that the full scale cavity length variations of sensor chip obtained by measurement and the FEM numerical calculation were generally coincident. The error of the full range cavity length variation was reduced to less than 13.4% when we designed the sensor chips with the FEM model. The maximum measurement range of the sensor was up to 105 MPa, and the measurement accuracy in the full scale was within 0.100%.