Optical fiber based distributed acoustic sensor (DAS) is one of the most attractive and well developed fiber sensing technologies in the recent decade. It can simultaneously detect and recover the waveforms of multiple mechanical vibrations along the sensing fiber with high sampling rate, providing abundant information of the environment, which has great potential applications in perimeter fence security, oil and gas exploration, seismic waveform detection, and other fields. This paper first reviews the principles involved in DAS system, including three types of reflectometry to locate the Rayleigh backscattering (RBS) along the long distance fiber, and the two methods to recover the vibration waveform using RBS signal. Then two typical applications of DAS system are reviewed, and finally the possible research trends of DAS technology are discussed.