To improve transmission line load capacity and prevent power grid accidents, an online sag monitoring system for overhead transmission lines based on spontaneous Brillouin scattering is designed using the principles of Brillouin scattering and coherent detection. Brillouin Optical Time Domain Reflection (BOTDR) technology is used to collect the strain signal distributed along the optical fiber in real time, extract the key features of strain sensing, and realize the accurate positioning and early warning of sag anomaly points. Laboratory data and substation test results show that the system can detect and accurately locate strain change information in real time. The strain monitoring system is simple in single-end measurement and can accurately locate the strain mutation point. It can realize the early warning of cable sag abnormalities, and can timely detect and deal with potential faults to ensure the safe and stable operation of the transmission line.