In this paper, we optimize the design of a fiber optic hydrophone through theoretical analysis. The result shows that the position of the fiber coils on the flat diaphragm and the number of the fiber layers have significant effects on the sensitivity of the hydrophone. The optimized position of the fiber coils and the number of the fiber layers are presented in this paper. The experimental result is in good agreement with the theoretical analysis, which proves this theoretical approach is effective for analyzing the sensitivity of flat diaphragm FOHs and optimizing the design and fabrication of FOHs.