Follow the Fukushima accidents, new nuclear safety objectives and safety requirements are being studied in many countries and organizations. As for China, beyond design basis accident and severe accident are integrated into design extension conditions (DEC) by New Revision of Design Safety of Nuclear Power Plants in 2016, which requires accidents under DEC to be identified and analyzed. To aim that, a practical method was developed for CAP1400 to concretely address the requirements. DEC-A cases are selected on the basis of engineering judgment, deterministic assessments and probabilistic assessment, and analyzed by safety analysis codes. 4 DEC-B cases and relevant sequences are identified on the basis of typical severe accident phenomena with some phenomena practically eliminated by CAP1400 design features; DEC-B mitigation measures are analyzed to be able to prevent core melt with high pressure, to maintain in-vessel retention, to control hydrogen risk, and to prevent containment overpressure. CAP1400 is the first NPP with passive safety systems to implement the concept of design extension conditions.