Abstract: Taking the steam generator of the primary circuit system of the PACTEL PWR thermal experiment device as the research object, the pressure drop and the relationship between heat transfer and fluid flow of the U-tube are established based on the equations of mass conservation, momentum conservation and energy conservation of the one-dimensional fluid flow model. Then, the flow distribution calculation program of the inverted U-tube steam generator based on genetic algorithm was developed, and the correctness and reliability of the program were verified by benchmark experiments. Finally, the flow distribution among the U-tube groups of the steam generator is calculated by using the flow distribution program, and the influences of the tube height, tube length and the heat transfer coefficient of the first/second side on the flow distribution within the steam generator are studied. The results of the developed flow allocator are in good agreement with the experiment. Under the selected natural cycle conditions, the backflow is more likely to occur in the long tube of the steam generator, and the backflow phenomenon is characterized by a wide distribution range and low single-tube flow rate. The velocity of the forward flow in an inverted U-tube is inversely proportional to the length of the tube and proportional to the height of the tube. The backflow velocity remained unchanged with the increase of pipe length and was inversely proportional to pipe height. When the coefficient of heat transfer is low, the total flow is inversely proportional to the heat transfer coefficient . When the heat transfer coefficient is higher than a specific value, the backflow occurs in part of the tubes and the total flow drops sharply.