Based on the mixed dilution theory and comprehensive attenuation theory of pollutants, a water quality model of river reservoir was established. According to the principle of material balance, a first-order kinetic reaction model of pollutant attenuation in inflow and reservoir storage was established. Taking Xiaolangdi Reservoir as an example, the model was tested by comparing the calculated values with the actual monitoring data. The test results showed that the average relative errors of the concentrations of ammonia nitrogen and COD were 19% and 15%. Through the established model, the water environmental assimilative capacity of Xiaolangdi Reservoir under different hydrological conditions was obtained.