Working principles of Fabry-Pérot optical fiber hydrophone is referred, and the reason for working point choosing and system solution are given. Under ideal circumstance, the resonance curve of Fabry-Pérot optical fiber interference is drawn. Stress birefringence of optical fiber and mode split within resonance cavity are analyzed by photoelasticity theory. With the theory of mode split, the distortion of resonance curve of hydrophone is explained and math formulas describing the distortion of resonance curve are given, and then the interference process of hydrophone after mode split is simulated by MathCAD and the theoretical curve is plotted. The result of experiments is consistent with the theoretical simulation. It shows that it is feasible to explain the distortion of resonance curves by the theory of mode split.