The invention discloses an online monitoring system for power distribution transformers. The online monitoring system comprises an upper computer, a master controller, a temperature sensor, a current detecting module, a voltage monitoring module, a power factor monitoring module and an alarm module; the upper computer is connected with the master controller, and the master controller is respectively connected with the temperature sensor, the current detecting module, the voltage monitoring module, the power factor monitoring module and the alarm module. The online monitoring system for the power distribution transformers has the advantages that voltages, currents and power factors are monitored in real time in an online manner, so that real-time load running conditions of the power distribution transformers can be mastered, bodies of the power distribution transformers and the temperatures of upper-layer oil of the bodies of the transformers are monitored in real time in an online manner via the wireless temperature sensor, the online monitoring system has a high-temperature alarm function, and faults of the power distribution transformers due to high temperatures can be prevented.