Phase sensitive optical time-domain reflectometer (Φ-OTDR) transmits light pulses into the sensing optical fiber, and uses the method of analyzing the back Rayleigh scattered light or forward scattered light of the sensing fiber to identify and analyze intrusion disturbance events. Compared with other distributed optical fiber sensing technology, Φ-OTDR can perform long-distance distributed multi-point measurement with high accuracy and reliability. The study of Φ-OTDR has important application value. In this paper, the basic principle, structure, system performance, signal demodulation, and application of Φ-OTDR are introduced, and the extension Φ-OTDR technology is prospected.