Abstract: In view of the problem that the actual flow capacity of the Sangequan inverted siphon is surplus while the Xiaowacao invert siphon is small, based on the engineering characteristics and the hydraulic design habit, frictional head losses of the Sangequan and Xiaowacao inverted siphons under different flows are analyzed and compared with the measured values. The results show that the head loss calculated by the Colebrook formula is closer to the measured value for the PCCP pipe and the steel pipe in the Sangequan inverted siphon, which is obviously less than the hydraulically designed value using the Chezy formula. For the Xiaowacao inverted siphon FRP pipe, the head loss calculated by Colebrook formula is closer to the measured value, which is significantly higher than the hydraulically designed value using the Chezy formula. On this basis, the roughness value and the equivalent roughness value are inversely calculated by the calculated frictional head loss. It is found that the deviation cause of flow capacity calculation is that the Chezy formula is not suitable for the flow pattern in large diameter inverted siphon pipelines.