The invention relates to a secondary equipment state online monitoring method. The method includes firstly acquiring data required to monitor of a process level and a bay level in a safe I area; sending the data required to monitor of the process level in the safe I area to a bay level IED to process, and then uploading the data required to monitor of the process level and the bay level to a monitoring master; transmitting self-detecting information of a data server and data network device of a station level to the monitoring master, and allowing the monitoring master to acquire own self-detecting information; and allowing a comprehensive application server to read at regular intervals and display the secondary equipment information of the safe I area in real time. By the aid of the method, information including self-detecting information, operating state information, alarming information and timing information of all secondary equipment of the station level, process level and bay level of an intelligent substation can be monitored, and comprehensive display and alarm of the secondary equipment can be realized; by the aid of the technical support provided by the method, producing efficiency can be improved, equipment shutdown times can be reduced, labor cost can be saved, and regular inspections and detections for secondary systems can be reduced or eliminated.