Abstract: Smart substation technology provides new methods and technologies for the operation and maintenance of relay protection. The online monitoring based on state information will be conducive to improve the reliability of relay protection system of smart substation and reduce maintenance workload. From the perspective of the composition of relay protection system of smart substation, the state information of relay protection and their relationships are analyzed. An online monitoring method based on all relationships among state information is proposed through adding logical calculation and comparison of real-time state information collected and uploaded from relay protection. Case study shows that some faults of relay protection can be detected by proposed method, which improves the effectiveness of operation and maintenance of relay protection system for smart substation.