Abstract: The encapsulation of ultrafine DFB fiber laser hydrophone probe was studied. A theoretical model of encapsulation structure was built by finite element method. The factors affecting the frequency response and sensitivity fluctuation of ultrafine fiber laser hydrophone were discussed. A balance was found upon sensitivity, frequency response consistency and other indexes. Finally, a fiber laser hydrophone was made. Its diameter was 6 mm, length was 55 mm, sensitivity was -130 dB and sensitivity response fluctuation between 100 Hz -2 kHz was 4 dB. The conformity of theoretical analysis and simulation was verified by experimental test.