After the Fukushima Daiichi accident, the new nuclear power units need to consider the design extension conditions according to the requirements of regulations, and corresponding standards have also been issued. However, the current list of design extension conditions does not include seismic accidents, and there is no clear requirement on whether the earthquake-resistance measures are required for design extension conditions. In view of the seismic risk not be ignored, HPR 1000 design requires that all design extension working conditions response measures should be designed according to SSE seismic resistance. This approach lacks pertinence and may also lead to over-design affecting economy. Based on the seismic probabilistic safety analysis (PSA) model of HPR 1000 reactor and the DEC-A sequence identified by the internal event PSA, this paper establishes the seismic PSA model for DEC-A operating condition, and conducts quantitative analysis on it, so as to determine the seismic requirements of DEC response measures, and improve the economy as much as possible while ensuring the probabilistic safety goal of nuclear power plant. In this paper, the seismic design method of DEC-A condition response measures based on seismic PSA is proposed for the first time at home and abroad, the seismic risk of nuclear power plants is evaluated comprehensively, and the seismic type of SSCs for DEC condition is determined. The methods and practices studied in this paper have effectively ensured the progressiveness and overall safety level of HPR series units, which can be applied to the research and design of new reactors in the future.