Abstract: A measuring system using compressed air as the working fluid was built in order to verify the flow resistance uniformity of the helical tubes of the experimental assembly for the high temperature gas-cooled reactor (HTGR) steam generator. The influence of the compressibility of compressed air on the measurement accuracy was carefully discussed. Two data reduction methods with linear average and integration were proposed, and the accuracies of the two methods were compared. Acceleration pressure drop should also be considered during data reduction, and its influence was analyzed under different working conditions. The accuracy and repeatability of the measuring system were first verified using a smooth straight tube and corresponding correlation. Then the flow resistances of the 9 helical tubes of the 5th layer of the experimental assembly were measured. The results show that the maximum relative deviation of friction factor among the 9 helical tubes is 5%. The main cause of the deviation comes from the deviation of the tube inside diameters. The flow transition Reynolds number of helical tubes is different from that of straight tubes. The flow transition Reynolds number of the 5th layer helical tubes of the experimental assembly is about 6000.