In view of the changes in communication structure, operating environment and quantity of information of the secondary system of smart substation, an online monitoring and assessing system structure of the secondary system is proposed. Such a system structure will make trend judgement and loss assessment of safety risks in smart substation through monitoring and evaluating its core parameters and operating conditions. Around 6 parts with protection and bay control unit, smart terminal, merging unit, HMI, time synchronization system and switch in the secondary system, the complete on-line monitoring and assessment model is described from three aspects — the evaluation parameter model, the evaluation sub-item model and the evaluation model, and finally the evaluation process of the on-line monitoring system is given. It can carry out the on-line monitoring and status assessment to the secondary system of the smart substation, bring great convenience for the maintenance and repair of the system and lay a foundation for performing the condition based maintenance and it also plays an important role in improving the safety and stable operation of the system.