Abstract: In order to evaluate the structural integrity of pressurizer surge line affected by thermal stratification, this paper presents the numerical simulation for thermal stratification of PWR NPP pressurizer surge line by using Computational Fliud Dynamics（CFD） method. The flow in the surge line is studied, and the heat transfer characteristics, flow field and temperature distribution are obtained and the relationship between the phenomenon of thermal stratification and the surge velocity is analyzed. The result shows that the maximal temperature difference increases along with the increasing of surge velocity, as the surge velocity changes within certain range. The maximum temperature difference of cross section and its locations with different surge velocity are obtained, and vulnerability of the surge line when thermal stratification occurrs is pointed out.