To study the runoff characteristics of small and medium-sized rivers, the Qinhuai River is selected as the research object. According to the topography, water system, production and confluence characteristics, water conservancy projects and downstream tidal level characteristics of Qinhuai River Basin, the hydrological and hydrodynamic model is established to calculate the water level and discharge process of the main sections in the Qinhuai River system. The K-nearest neighbor algorithm (KNN) and the simulating feedback method are selected to correct the simulation results. The results show that the hydrological and hydrodynamic model of the Qinhuai River Basin has a good simulation effect by taking the relative error of flood peak and Nash-Sutcliffe efficiency coefficient as evaluation indexes. The simulation accuracy is further improved after the real-time correction, which enriches the prediction model of medium and small rivers.