In order to solve the existing problems and shortcomings of the traditional relay protection maintenance mode, the research work of online condition based maintenance of relay protection is carried out. First of all, the online protection equipment working state data is collected through the information system, providing data support for the relay protection status evaluation. Secondly, it establishes the state evaluation and intelligent decision information model of relay protection equipment, and carries on the on-line state evaluation to the key link that affects the relay protection function. Then, equipment condition based maintenance is carried out according to the evaluation results. Finally, the implementation method and state evaluation strategy, such as on-line detection temperature humidity of relay protection operating environment and transceiver power state of optical fiber loop, are also proposed. The maintenance practice in a provincial power grid proves that the on-line condition based maintenance of relay protection can improve the efficiency of relay protection maintenance and the safe operation level of relay protection equipment.