The utility model discloses an on-line monitoring device for a power transmission transformer. The system comprises a temperature sensor oil chromatography on-line monitoring cabinet, a carrier gas cylinder, a pressure sensor, a discharge signal acquisition module and a discharge signal processing module which are connected with a local on-line monitor, and the local on-line monitor is connected to a remote power center monitor and a handset through wireless signals; the oil temperature of the transformer can be detected in real time through the temperature sensor; the oil chromatography on-line monitoring cabinet can analyze gas components and corresponding concentrations in the transformer oil; possible fault hidden troubles of the transformer can be analyzed in time; the discharge signal acquisition module and the discharge signal processing module jointly complete detection of partial discharge of the transformer. According to the utility model, the transformer fault caused by partial discharge is eliminated, various detection information can be checked on site in the local online monitor, and can be remotely checked through the remote power center monitor and the handset, so that a remote maintenance strategy is provided, and the safety and stability of power utilization are ensured.