Based on improved graph theory, the river network was generalized by a graph model. Considering the influence of water quantity on the hydrologic connectivity of river network, the connectivity factor reflecting the water quantity index was taken as the weight and the quantitative evaluation model of hydrologic connectivity degree was built. MATLAB was used to calculate the river network weighted connectivity degree, and the quantitative evaluation of river network hydrologic connectivity was conducted. Taking the Qinhuai River Basin as an example, the HEC-HMS hydrological model was established and the short-term and long-term floods were simulated. The connectivity degree of the river network in the basin under different scale floods was analyzed. The connectivity was graded according to the evaluation results. The results show that the flood volume and the connectivity degree are basically linear. The short-term flood has a poor connectivity degree compared to the long-term flood. The evaluation model has good applicability and reliability.