The invention discloses a power transmission tower online monitoring system and monitoring method. The power transmission tower online monitoring system comprises a signal acquisition device, a signal processing station and a signal storage and processing center; the signal acquisition device is arranged on a power transmission tower; the signal acquisition device acquires the vibration information of the power transmission tower and the meteorological information of a location where the power transmission tower is located and transfers the acquired information to the signal processing station; the signal processing station is arranged in a range which adopts the power transmission tower as the center and a predetermined distance as the radius; the signal processing station collects the information acquired by the signal acquisition device, pre-processes the information and transmits the pre-processed information to the signal storage and processing center; and the signal storage and processing center receives the information pre-processed by the signal processing station, analyzes and stores the information. The power transmission tower online monitoring system and the power transmission tower online monitoring method are used cooperatively, and the abnormal situations of the power transmission tower can be discovered in time. The power transmission tower online monitoring system and monitoring method are simple and efficient in monitoring process and are excellent in monitoring effect, and can actually realize unmanned monitoring.