The invention relates to an online monitoring system for a high-voltage reactor. Four sensors are mounted on four walls of the reactor and are connected with a signal acquisition unit; the signal acquisition unit is connected with a signal regulating unit; the signal regulating unit is connected with a center server; the center server in a master control cabinet is connected with a display; a vibration signal is converted into a voltage signal by the four sensors and is sent into the signal acquisition unit; the regulation and acquisition for the signal are performed by the signal acquisition unit, thereby obtaining original real-time data for reflecting a vibration state of the reactor 1; and according to a TCP/IP (Transmission Control Protocol/Internet Protocol), the data are sent to the center server and are further analyzed and stored, thereby generating various specialized analysis maps for an on-site running person to look up and analyze the vibration data, know the vibration state of the reactor at any time and prevent accidents. In the online monitoring system, a mechanical vibration principle is utilized to on-line measure the vibration of the high-voltage reactor. The online monitoring system has the advantage of low running and maintaining costs.