The change of channel storage capacity is an important content of river evolution analysis. In the past, the section method was mostly used for simplified calculation. However, in the complex river pattern with multistage branching or tributaries, the section method often needs to be dealt with separately, and the calculation efficiency is low. By introducing the water surface width ratio of the branching channel, optimizing the distribution of the calculated area of the section at the branching point, adding the section, estimating the storage capacity of the channel in the missing section near the tributary estuary, and using the topographic method to restore the section spacing under different water levels, the calculation model of the section method under the complex river pattern is improved. The comparison results show that the calculation accuracy of tank storage capacity is greatly improved by using the above method, which can provide a reference for batch calculation of tank storage capacity under various complex river types.