According to the characteristics of river flow and the calculation precision and efficiency of the model，the one-dimensional flow mathematical models of the Yangtze River usually divide the river channel below Yichang into two sections with Datong as a boundary. The runoff model is built from Yichang to Datong and the tidal current model is built from Datong to Liuhekou. However，under new water and sediment conditions and complex river and lake networks，the traditional two-section model can hardly simulate the long river section of complex river networks as a whole. In response，a one-dimensional flow mathematical model for the river network is built based on the high-precision and high-performance finite volume method. It solves the interface flux of adjacent units by SLIC and simplifies the calculation of coefficient matrix with the control body as the calculation unit for the intersection of complex river networks. The physical process of the model is clear with convenient and fast calculation. The one-dimensional flow mathematical model is applied to the section from Yichang to Liuhekou. The results show that the model is stable with high precision and effectively realizes the digital-model integration of the trunk line of Yangtze River below Yichang.