To reduce the fluctuation of frequency response of a Distributed Feedback (DFB) fiber laser used in underwater sound detection, an encapsulated structure with orifices was designed.With the encapsulated structure, both ends of the DFB fiber laser were fixed along the axes of the structure by the tension of polyurethane,and the fluctuation of mid-frequency response was restrained. The dynamics of the encapsulated structure was numerically analyzed with ANSYS, and a prototype of the new kind of hydrophone based on the structure was produced and tested with the vibration fluid column method. The results show that the fluctuation of frequency response of the DFB fiber laser is restrained effectively from 20 Hz to 800 Hz, and the acoustic pressure sensitivity reaches to -131 dB with the fluctuation less than ±1.5 dB. These results demonstrate that the performance of the DFB fiber laser hydrophone has been improved by the protection of the encapsulated structure and the tension of polyurethane.