Abstract: Lying at the junction of the Nenjiang River, the Songhua River and the second Songhua River, situated at the upstream of Harbin city, the Pangtoupao flood retarding basin plays a key role in alleviating the Harbin city flood pressure. The flood discharging capacity and its influencing factors of the Laolongkou flood diversion sluice are studied by the mathematical model calculation and physical model tests in this research. The mathematical model is applied to analyzing the evolution of the channel erosion or deposition and the variations in the water level of the Nenjiang River, and giving the import and export boundary conditions for the physical model. And the physical model is applied to studying the discharging capacity of the flood-diversion sluice under different given schemes. The analysis results show that the discharging capacity of the Laolongkou flood diversion sluice mainly depends on the water levels of both the Nenjiang River and the diversion channel, and is very sensitive to their changes. The influencing factors of the flood discharging capacity of the sluice mainly include the changes of erosion or deposition of the Nenjiang main stream, the flood diversion discharge, the flood type, and the direction of the diversion channel, etc. Specifically, as for as the Nenjiang water level concerned, the variation of scouring and silting in the main stream of the Nenjiang is very small, and has little effect on the water level of the main stream. The influences of the flood diversion discharge on the water level of the main stream of the Nenjiang River aremore obvious, and the water level drops to 0.70 m during the maximum flood diversion discharge. As for as the Pangtoupao basin water level, the influence from the type of the simultaneous flood originates from the Nenjiang River, and the second Songhua River is greater than the flood singly originating from the Nenjiang River. Owing to the complex topography in the flood retarding basin, the different flood diversion channels have significant and different impacts on the water level of the flood diversion sluice, which is the key factor affecting the flood discharging capacity of the sluice. Therefore, it is very necessary to set upflood diversion channels in the flood retarding basin for rapid dispersion of flood.