Abstract: The long fuel cycle will be implemented in Tianwan Nuclear Power Station, and the design basis accident needs to be re-analyzed since the change of core design. The large break LOCA was analyzed in the paper. Based on the input data and calculation assumption, the most conservative calculated case was obtained through the conservative analysis of the axial power distribution and the ECCS. It is shown that the accept criteria are satisfied in the large break LOCA on long fuel cycle, and the safety margin of the core design is large enough.