Complex changes in flow division ratio can be aroused by channel morphological adjustments in anabranching reaches. Establishing a precise calculation formula of flow division ratio lays a foundation for investigating the changes in flow division ratio under different flow-sediment and channel boundary conditions. Taking the typical anabranching reach, namely the Shashi anabranching reach in the upper Jingjiang River as the case study, this paper compares the applicability in natural reaches of different flow division calculation formulas based on data observed after impoundment of the Three Gorges Reservoir (TGR) in 2003, and proposes an improved flow division formula with higher accuracy by introducing a comprehensive factor which can reflect the differences in roughness and water surface gradient between the primary and secondary branching channel. Based on this formula, the variation characteristics of flow division ratio at the study reach are further analyzed. The results reveal the following points:(1) Without accurately reflecting the differences in roughness and water surface gradient between the primary and secondary branching channels, the calculation error in low flow division ratio result is large and the maximum absolute difference (MAD) can reach 15%. (2) The average water depth ratio of the primary to secondary branching channel is highly related to their roughness-gradient comprehensive influence coefficient ratio and by introducing the average water depth ratio, a flow division calculation formula with a significantly improved accuracy of less than 5% MAD is established. (3) After impoundment of the TGR, the channel scouring and silting adjustments exert greater impacts on the low flow than high flow division ratio and led to a more significant variation of flow division ratio in the slightly curved branching reach than that in the straight branching reach.