When a Distributed Feedback(DFB) fiber laser is used in underwater acoustic detection,it is found that the acoustic pressure sensitivity of the DFB fiber laser is higher than that calculated under hydrostatic boundary conditions,and its frequency response curve is not smooth.To explore the reason and the mechanism of the above,the dynamical characteristics of the DFB fiber laser are investigated.Firstly,the vibration models of the DFB fiber laser whose both ends are fixed and placed in air and water respectively is analyzed numerically.And then,with specially designed testing assemblies,experimental study on it is also conducted.The results indicate that the first order natural frequency of the DFB fiber laser is related to the length of the optical fiber between the two fixed ends,and the frequency decreases with the length increases.When the length is set as 50,55 and 60 mm respectively,the first order natural frequencies have been found to be about 250,200 and 125 Hz in air,and 200,160 and 120 Hz in water.These data show that when vibration takes place in the DFB fiber laser, its output will be changed to cause the fluctuation of acoustic pressure sensitivity,which shows the first natural frequency is an important factor for the underwater acoustic detection.