Abstract: The experimental study on reverse flow in the inverted U-tube steam generator was carried out. Based on experiment data, the experiment phenomenon was simulated by CFD method. The results show that reverse flow causes remarkable temperature stratified in inlet plenum and reduces the heat transfer capability of the steam generator U-tubes. The absolute value of pressure drop decreases in plenum when reverse flow occurs. The flow resistance increases while velocity of normal U-tubes increases, which has adverse influence on natural circulation.