In order to improve the acoustic sensitivity of a fiber optic hydrophone in a deep-sea environment， a deep-sea fiber optic hydrophone element based on a push-pull mandrel structure was designed. It could effectively detect target underwater acoustic signals of 10 Hz-2 kHz at 3 000 meters in the deep sea. First， based on the Michelson interferometer， the composition and acoustic sensitivity of the fiber optic hydrophone with a push-pull mandrel structure were introduced. Then， the acoustic pressure conversion mechanism was analyzed using elastic mechanics theory. Afterward， the influence of the elastic tube structural parameters of the hydrophone primitives on acoustic sensitivity was simulated via the finite element method. Meanwhile， the working stability under deep-sea conditions was explored. According to the calculation results， five groups of fiber optic hydrophone elements with different structures and manufacturing processes were fabricated. Finally， the working performance of the five groups of fiber optic hydrophones at 3 000 meters deep-sea was tested using a high hydrostatic pressure acoustic sensitivity system to verify the influence of structural parameters and manufacturing processes on acoustic sensitivity in a deep-sea environment.The experimental results show that the structure parameters and fabrication process of fiber optic hydrophone can significantly affect its working performance and stability in deep-sea environment. After optimization， the fiber optic hydrophone's response of underwater acoustic signal in 10 Hz-2 kHz frequency band at 3 000 meters deep-sea can reach -127 dB re rad/μPa， which is 7 dB re rad/μPa higher than before. It basically meets the requirements of high sensitivity and reliable detection of sound waves for deep-sea application.