Factors such as lake reservoir inflow runoff, reservoir surface evaporation, atmospheric dry and wet sedimentation, precipitation in the reservoir area, sedimentation self-purification, water temperature fluctuation, uneven mixing, storage capacity change, etc. will all affect the size of the water environmental capacity of the lake reservoir. To determine the quantitative impact of each factor, a daily dynamic calculation model of water environmental capacity was established for the calculation, and the factors were divided into physical factors, calculation parameters, and reservoir management objectives according to their input characteristics. Then the calculation results were compared and analyzed with the capacity benchmark value obtained from full factor analysis. The results show that among physical factors, the inflow runoff has the strongest correlation with TN capacity results and the precipitation in the reservoir area has the greatest impact on the deviation of TN capacity results under the basic design hydrological conditions. Correction factor of water temperature α in calculation parameters has the great impact on TN capacity, an increase of 5% corresponds to a bias value of 8.27% in TN capacity. Water quality management objective in reservoir management objectives has the greatest impact on TN capacity and is the most influencing factor of all factors. The quantitative analysis clarifies the influencing degree of each factor on the capacity accounting result, and provides a technical support for simplification and improvement of the capacity accounting method.