To reduce the fluctuation of a distributed feedback (DFB) fiber laser used in underwater sound detection, a fiber laser hydrophone of encapsulated structure with two diaphragms is designed. This structure increases the activated area of the acoustic pressure through the diaphragms, which is transformed to the axial strain in the fiber laser to realize the enhancement of the pressure sensitivity. After the structure is analyzed by ANSYS, the prototypes of the new kind of hydrophone based on the structure are fabricated and tested. Experimental results show that acoustic pressure sensitivity of the DFB fiber laser hydrophone reaches to -136.9 dB with the fluctuation less than ±0.5 dB in the frequency range of 2.5~10 kHz, which shows that the working frequency of the hydrophone is expanded to high frequency and it can well meet the requirements for the engineering application of flank array.