Taking the Herong station as an example, this work analyzes the swing range of the water level and flow rate by the systematical method. The result shows that there is a certain law in distributions of axial swings for the flood-peak water level and the corresponding flow rate under the impact of flood fluctuations. This law gets stronger when the relationship of water level and flow rate is closer to the flood peak. Meanwhile, the water level forms a regular series with the axis, which can establish the relationship between the flood-peak water level and the corresponding flow rate, and that between the water level and the flow conversion model of the flood peak. This study effectively solves the bottleneck of flood level forecasting under the impact of flood fluctuations on the relationship between water level and flow rate, for meeting the requirement of flood control.