Abstract: The thermal stratification under transient condition in pressurizer surge line of Qinshan Phase Ⅱ extension nuclear power project (2×650 MW PWR generator 4) was investigated by computational fluid dynamics program ANSYS/CFX. The whole and cross-sectional thermal stratification transient analysis models for the pressurizer surge line were established, and the heat stratified flow and heat transfer of the surge line were studied. The way of temperature growth is different between high- and low-temperature fluid layers in the same cross section. The fluid temperature distribution has great difference in different cross sections, but the temperature difference first -increases and then decreases in every cross section. The research results can provide a basis for subsequent analysis of thermal stress and lifespan.