There are noteworthy problems in current strategies to calculate river water environmental capacity(WEC), including the generalization of tributaries and water intakes, which results in inaccurate calculation results of the WEC, and the difficulty in adapting to dynamic changes of demands and hydrological conditions in terms of practical application. To address these flaws, the subsection summation model(SSM) was built for river WEC calculation. The SSM increases the number of control sections according to drain outlets, water intakes, and tributaries and acquires the WEC of the functional area section-by-section. The Wei River was taken as the study area for verification and application of the SSM. Supported by a comprehensive integration platform, the WEC simulation system of the Wei River was constructed. The results show that the SSM enhances the accuracy of the WEC calculation, and the results are closer to the actual situation. The simulation system could obtain the WEC according to the demands and changes in the hydrological conditions, thus providing technical means for policy-makers.