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Aiming at the application of ultra-long-distance monitoring in scenarios such as power transmission lines, this article uses Raman amplification and heterodyne detection based on a distributed optical fiber phase sensitive Optical Time Domain Reflectometer (OTDR) to design a photoelectric signal segmented detection collected optical fiber sensing system. According to the length of the optical fiber, the optical fiber is divided into two sections for signal processing and acquisition, and the corresponding signal processing and acquisition parameters are adapted to different strength signals to solve the problems of insufficient dynamic range and poor signal-to-noise ratio of the ultra-long-distance monitoring system. After testing and analysis, a single-end ultra-long-distance distributed optical fiber monitoring system with a distance of more than 124 km is realized. The signal-to-noise ratio at the end of the system is larger than 10.5 dB.