Abstract: A method for determining a contribution rate of pollution load in a water quality assessment section of an annular river network system based on water quantity constitute comprises the following steps: defining all water quantity components (rainfall, pollution discharge and water diversion) according to a water quantity source condition of a river network water system in a research region; calculating a water quantity ratio of each water quantity component in each water quality assessment section; collecting flow rate of point source pollution and corresponding pollutant discharge amount, calculating a water quantity weighted average concentration of all point source pollutants, and calculating a runoff rate and a pollution load of all land use types and an average concentration of non-point source pollutants by utilizing a hydrological model and a pollution load model; and calculating the contribution rate of pollution load in the water quality assessment section.