The new characteristics and new changes of smart substation put forward new requirements for the condition based maintenance of relay protection in smart substation. The common information model of the secondary equipment of smart substation is the foundation of the construction of new smart substation relay protection condition based maintenance. IEC 61970 and IEC 61968 Public Information Model (CIM) can not provide comprehensive public information model support for relay protection condition based maintenance in smart substation. Based on the basic principle of extension of the IEC 61970 and IEC 61968 CIM, through the new architecture through the analysis of the secondary loop network information interaction of relay protection device and the physical structure of hierarchical smart substation protection and control system, this paper extends the protection secondary equipment resources of public information model and secondary relay protection equipment assets of public information model of smart substation relay. It proposes a model mapping scheme from the secondary equipment state information model of substation based on IEC 61850 to the secondary equipment measurement model of smart substation relay protection, and constructs a complete common information model of secondary equipment for relay protection in smart substation. These lay the foundation for the application of condition based maintenance for smart substations and the integration and interaction of information in the secondary operation and maintenance management systems. This work is supported by Science and Technology Project of State Grid Corporation of China “Technical Research and Application of Relay Protection Status Maintenance in Smart Substation” (No. 521200160021).