Being safe and highly efficient is the core requirement of the smart grid construction. This paper summarizes the studies on the state assessment of relaying protection system. The state division of relaying protection system, diagnostic methods of hidden failure and risk assessment are focused. Based on the comparison of the advantages and disadvantages of all the studies from the aspects of diagnostic methods, assessment models, indexes of risk evaluation and fundamental data source, this paper points out new indexes of relaying protection system state evaluation, real time diagnosis technology and new standard for information interface will be the main issue of research in the future.