Abstract: It is necessary to use the computed fluid dynamic（CFD） method to study the problems considering the U-tube distribution, such as the reverse flow in the steam generator. But it is difficult to model all the U-tubes in a steam generator because of the huge number of the U-tubes and the limited capability of computers. This paper discussed a simplified modeling approach for single-phase fluid flow in U-tubes, using a square tube instead of a round tube. The result of a reverse flow calculation for a steam generator using the simplified modeling approach is basically the same as the result using a non-simplified model, which indicates that the approach can be used in CFD analysis of this kind of problems.