The equilibrium condition of island morphology is of great importance to the understanding of the self-adjusting mechanism of anabranching rivers. Based on the basic flow relationships of continuity, resistance, sediment transport and energy conservation, the equilibrium condition of a two-channel anabranching system with the island planform of approximately a triangular form is derived theoretically by applying a variational approach. The results show that when the anabranching river achieves stationary equilibrium, the main channel with a larger carrying capacity takes a larger proportion of sediment load than the proportion of flow discharge. The shape of island morphology in equilibrium varies with the proportions of flow discharge and sediment load passing into the two channels. The smaller the discrepancy between the proportions of flow discharge and sediment load, the slenderer the island shows. In addition, for Jianli anabranching reach located in the middle Yangtze River, it is demonstrated that the theoretically computed morphologies of the island are highly consistent with the field observations, with a relative error of about 9.0%.