The HPR1000 nuclear power plant (NPP), as an advanced generation III nuclear power technology in China, has implemented several specific measures and systems for design extension conditions (DEC) mitigation. To ensure the mitigation capability and safety of HPR1000, a comprehensive demonstration of equipment availability under design extension conditions is necessary and the analysis method is different with generation II + NPP because of its new design. In the practice of HPR1000 for severe accident equipment availability, the equipment is divided into two categories and a general principle has been adopted to ensure the availability of “necessary to use” category equipment through severe accident equipment qualification (EQ) and for “may be used” category equipment, availability assessment and evaluation is conducted. This paper studies the equipment and instrument lists involved in the analysis process, the operational time window of equipment and instruments, the accident environmental conditions and qualification conditions analysis and calculation, and the methods of availability demonstration, laying an important foundation for the development and implementation of severe accident management guidelines (SAMG). In addition, after the revision of IAEA SSR-2/1 and HAF102 in 2016, new requirements for the availability assessment of design extension conditions without significant fuel degradation (DEC-A) were added. This paper proposes a process for DEC-A availability demonstration: “DEC-A scenario sorting - identification of mitigation functions and measures - commonality analysis with design basis accident/severe accident-DEC-A availability assessment,” which improves the ability to mitigate DEC accident and enhances the safety of nuclear power plants.