Abstract: To ensure reactor protection system of nuclear power plant (NPP) meeting the nuclear safety and user requirements, the key characteristics of the reactor protection system based on NuPAC for China advanced large commercial NPPs were analyzed. Fourteen key characteristics were identified, and these key characteristics cover the requirements from laws, regulations, codes and standards, e.g. single failure criteria, independence, integrity, quality, diversity, reliability, security, operability, maintainability and performance, and so on, as well as the important requirements of the end user, e.g. compatibility, design margin, sustainability, flexibility and economy, and so on. The key characteristics acquired after analysis provide good basis and instruction to the requirement analysis for reactor protection system.