The invention relates to a distributed sag online monitoring system and a distributed sag online monitoring method for an optical fiber composite overhead ground wire. The online monitoring system comprises a distributed optical fiber temperature and strain interrogator, a distributed optical fiber sensor and a computer system and information processing unit, wherein the distributed optical fiber temperature and strain interrogator is installed in a high-tension transmission line transformer station, the distributed optical fiber sensor is a sensor optical fiber in an optical fiber composite overhead ground wire, the sensor optical fiber of the optical fiber composite overhead ground wire is connected to the distributed optical fiber temperature and strain interrogator through an interface in the transformer station, and the distributed optical fiber temperature and strain interrogator is connected to the computer system and information processing unit. According to the distributed sag online monitoring system and the distributed sag online monitoring method, distribution of the sags of each span in the whole line can be monitored, system reliability is strong; online monitoring is adopted, thus being good in real-time performance and high in efficiency; the main machine of the detecting device is placed in the transformer station so as to be less influenced by environment; no additional sensor is arranged on the scene, thus being easy to maintain.