Since the phase change of the scattered light is extremely sensitive to external disturbances, the phase sensitive optical time-domain reflectometer (Φ-OTDR) technology has the characteristics of high response speed and ultra-high sensitivity. Compared with other existing distributed optical fiber sensing technologies, Φ-OTDR technology has obvious advantages in environmental adaptability and convenience of optical cable layout. This article introduces the principle, structure, performance, and application of the Φ-OTDR system, and looks forward to its development trend.