This paper introduces the application background for developing an laser Doppler vibrometer (LDV) for underwater acoustic communication and expounds the principle of laser Doppler vibrometer and two coherent detection methods. Using the way of homodyne coherent detection, a fiber laser Doppler vibrometer is designed and completed. To prove the system can be used in underwater acousto-optic communications, with this system, the acoustic frequency and intensity of underwater sound source are detected. Through the analysis of experimental data, two conclusions are gotten: first, the system is able to detect the acoustic frequency of underwater sound source, the standard deviation of measuring ten acoustic frequencies near the frequency of 7 kHz is less than 8 Hz; second, the signal intensity of system detection is an exponential function of acoustic press level of underwater sound source. For the acoustic band of 3.5 kHz and 7 kHz used in underwater communications, the smallest sound press level of detection reached 146.2 dB and 150.8 dB respectively.