To solve the problem that fluid-structure coupling mode makes the frequency band of hydrophone narrowed, the influence of fluid on the natural frequency of the distributed feedback fiber laser hydrophone with two diaphragms was theoretically analyzed. Based on the finite element software, the mode and frequency response performance of the hydrophone in the air and fluid before and after the reinforcement were simulated. Prototypes of the hydrophone were fabricated and tested. The results show that response curve of the hydrophone before reinforcement fluctuates greatly due to the existence of several natural frequencies in a frequency range of 2 500~8 000 Hz. The average acoustic pressure sensitivity of the hydrophone reaches -142.77±0.8 dB after reinforcement, which verifies the influence of fluid on the frequency response performance of the hydrophone and the effectiveness of the measures. It agrees well with both the theoretical analysis and simulation results.