Abstract: High water level in the downstream can be a key factor that influences the drainage capacity of the stormwater pipe. This can form the hydraulic jacking that can substantially affect the operation and prevention of urban flooding. However, there is a lack of understanding on the hydrodynamic mechanism of such an interacting process. To this end, a set of physical experimental tests were conducted on a tank-pipe-channel laboratory system. The results obtained show that downstream jacking at the early stage (under non-full flow conditions) can enhance pipe flow capacity up to 18.7% as a result of suction effect and the increased flow area. This is followed by a significant block on the drainage capacity of the pipe due to the excessive jacking with a maximum flow reduction about 20.3%. In addition, the flow in the downstream river can facilitate the pipe drainage under the jacking conditions, and such an effect can be stronger as the river flow velocity increases. For such scenarios, experiment tests show that the maximum increase of total pipe drainage and peak flow can reach 4.6% and 8.5% respectively. These findings significantly improve the understanding on the hydrodynamic mechanism between high downstream water level and the pipe drainage process, which can offer important scientific guidance for the regulation of urban river networks and the design of drainage pipe systems.