Abstract: For natural circulation, it is shown that reverse flow occurs in inverted U-tubes of steam generator (SG). The flow and heat transfer characteristics in inverted U-tubes were analyzed, the principle of the reverse flow was gotten, and the flow distribution program was established. The flow distribution of the parallel inverted U-tubes in one type of SG was calculated using the program, the calculation results were compared with experimental data, and the validity of the program was verified. The change of the parallel U-tubes’ parameters with the pressure drop was also calculated and studied by using the program. The results show that reverse flow occurs in the short tubes and the net mass flow rate and the heat transfer rate in the primary side are lowered down step by step, which has great influence on the reactor safety.