Abstract: The sealing of check valves as the NPP primary circuit pressure boundary extension is directly related to the leakage and integrity of the primary circuit, which requires periodic tests to ensure the safe operation of the reactor. The relevant ASME flow rate standard is generally used as the acceptance criterion for leakage periodic tests. However, the leakage rate of check valves cannot be accurately measured, so it is necessary to convert the flow rate criterion. Meanwhile, the corresponding and matching acceptance criterion is required to be designed due to the different sizes of check valves and various layouts of associated pipelines.This article takes the check valves as the primary circuit pressure boundary extensi on of safety injection system of China’s advanced PWR as an example. Based on the leakage test principle of check valves and combined with engineering experience, a generally applicable method of the acceptance criterion design for the check valve leakage is proposed, starting from the criterion conversion and through rigorous analysis, which fills the blank in the design method study.