Pressure phase sensitivity is one of the most important performance indexes for the fiber-optic hydrophone. Maximizing the pressure phase sensitivity is a basic principle in the fiber-optic hydrophone design. In this work, the orthogonal experimental method was applied to optimize the design of the pressure phase sensitivity of the fiber-optic air-backed mandrel hydrophone. A general three-dimensional (3D) theoretical model was created, and nine fiber-optic hydrophones were designed and made for orthogonal experiment. Several factors which affect the sensitivity were analyzed in the experiments, and the results validate the theory. The optimized designs are also gained by the analysis of the results to maximize the pressure phase sensitivity.