Abstract: The Safety Performance Indicator（SPI） for nuclear power plants has been established by National Nuclear Safety Administration of China（NNSA）, which is used for evaluating plant safety conditions and implementing corresponding regulatory measures. The Safety System Unavailability Indicator（SSUI） is a part of SPI, which is utilized to evaluate the availability of important accident mitigating systems when demanded. Specific systems and SSUI evaluation criteria are not completely applicable for AP1000 due to its unique design features. In this paper,risk-informed decision making methodology is adopted. Firstly, the systems in AP1000 whose functions are determined in the SSUI are identified, and sorted by system importance that is quantified in the probabilistic safety analysis（PSA）. Systems that shall be included in the SSUI are selected based on the result of sorting. Then the risk increase on the unavailability of the class1 E DC and uninterrupted power supply system（IDS） is quantified and correlation between the safety level and unavailability are determined based on the relation between the risk increase and the safety level.