A laser coherent detection system with wavelength of 1 550 nm was established. The stability of the system has been improved because of the all-fiber optical path. The underwater vibration signals with different frequency, intensity and depth have been studied in experiments. The results show that the fluctuations of water surface are detected by the laser coherent detection system. The vibration characteristics of the underwater acoustic field can be extracted effectively by the time-frequency analyses. Experimental results indicate that the detection system can be used to detect real-time underwater acoustic signals at frequency from 40 Hz to 10000 Hz with test error less than a few Hz. Therefore the laser coherent radar can be used for feature extraction and recognition of underwater target in real time.