Due to the problem that the small-scale physical model of the long-distance flood diversion tunnel is difficult to provide similar resistance conditions with the actual situation, we carried out a small-scale physical model test on the flow capacity of the inlet gate chamber section and the delivery capacity of the tunnel section of a flood diversion tunnel in a county of Shanxi Province. Based on the characteristics that uniform flow can be formed in the tunnel section, the calculation method of open-channel uniform flow is used to verify the measured results. The results show that the minimum difference between the calculated flow and the measured flow is 11.71 m3/s, and the relative error is 1.93%. Because the distance of the entrance gate chamber section is short and the local loss is far greater than the on-way resistance loss, the water depth at the stake number 0+099.25 m of the tunnel is controlled to the normal depth of the tunnel to directly verify its flow capacity; the results show that the maximum error of the calculated flow rate of the entrance gate chamber section does not exceed 1.78%. In the case of the flood once in 30 years, the flow capacity of the gate chamber and tunnel of the recommended scheme are basically consistent with the measured values. The flood diversion flow can reach as high as 863 m3/s, which is 63 m3/s higher than the designed overflow of 800 m3/s, accounting for 7.88% of the designed overflow, indicating that the recommended scheme can fully meet the design requirements.