In order to improve the precision of the simulation of small and medium-scale floods when using traditional methods, we used a method of doing hydrology backwards to simulate the flow at the outlet cross-section of the study area. Based on the characteristics of slowly-varying low flow in the study basin, we deduced the relationship between the average runoff of a period and the changing rate of the average runoff using the water balance equation, established an equation for the simulation of small and medium-scale floods based on doing hydrology backwards, and deduced the application range of the equation. The simulation of floods over the Qingjiang River Basin during the period from 1989 to 1995 showed that the simulated rising and recession trends agreed with observed data, the recession curves were better fitted than the rising curves, and the precision of the simulation of flood peaks met the requirements. Compared with traditional flood simulation methods and low-flow forecasting methods, the proposed method can be easily used for calculation and can describe the magnitude of peak flows of small and medium-scale floods using flood hydrographs, providing a scientific decision-making basis for reservoir operation.