Abstract: After the long fuel cycle is adopted at Tianwan Nuclear Power Station (TNPS), the core thermal and physical parameters will be changed, thus the conclusion in Final Safety Analysis Report is not applicable, and therefore the accidents need to be reanalyzed. In this paper the main assumptions and analytical methods were presented, and the transient calculation program of DINAMIKA-97 was used in analyzing the loss of flow accident. The analysis results indicate that all the safety criteria are met for the loss of flow accident, and the nuclear power station safety can be ensured.