Abstract: The heat transfer efficiency in high-multiplication water-cooled solid breeder is directly affected by the characteristics of the flow distribution in parallel tubes, however, there are no simple and effective methods for the initial design of the parallel tubes. The characteristics of flow distribution in U-type parallel tubes are simulated by Fluent program, and the sensibilities of geometry parameters and Reynolds number of the tube are analyzed. After that, the key parameters that affect the characteristics of flow distribution are obtained, including the channel length, channel area, number of channels as well as manifolds area. And a method for predicting characteristics of flow distribution is provided. Analysis results show that the lower the dimensionless factor Kfd is, the better the characteristics of flow distribution is. When Kfd is lower than 0.03, the flow loss ratio will be less than 5%.