Abstract: Thermal stratification is an asymmetrical temperature distributing phenomena which is due to the hot liquid and the cold liquid which have low velocity are not mixed. There are thermal stratification phenomena in the pressurizer surge line. It’s of great importance for the safety operation of nuclear power plants to research the thermal stratification in the pressurizer surge line. This paper theoretically analyzed the origin for thermal stratification, and numerically simulated the thermal stratification phenomena. The stratification transients are completed in different cross-sections of the surge line.