Abstract: The numerical simulation to U-type momentum regaining parallel manifold was completed with Realizable k-ε model by means of the ANSYS FLUENT software. An analytic method for the optimal cross section area ratio of the header (OARH) was obtained on the basis of the pressure change within the headers. The results show that the cross section area ratio of the dividing header to the combining header has a large effect on the flow distribution in the manifold, and the OARH obtained by both numerical simulation and analytic method is about 0.6. The method to match header can enhance the flow distribution uniformity in the parallel manifold of heat exchanger, and offer a guide for the design of the manifold of heat exchanger.