Abstract: For the reverse flow to occur within partial U-tubes of U-tube steam generator (UTSG) under natural circulations, a new asymmetric U-tube was proposed to enhance the natural circulation capacity and ease the reverse flow. Based on the basic conservation equations, the mathematical model was developed for calculating the pressure drop in U-tube and elevation difference between cold and heat sources. On this basis, taking a nuclear power plant U-tube as prototype, the changes of critical mass flux and elevation difference between heat and cold sources were calculated before and after improvement. The results show that the new asymmetric U-tube can reduce critical mass flux significantly and increase the elevation difference between heat and cold sources. The appropriate adjustment of the length ratio between up section and down section can ease reverse flow and improve the natural circulation capacity.