The objective ecological flows in vital controlled sections in main stream and tributaries of the Hanjiang River Basin are calculated with hydrological methods. Based on graph theory, the river network graph model of the Hanjiang River Basin was constructed according to the characteristics of the river system, the distribution of important water conservancy projects and control sections. Flow at each node of the river network was determined by carrying out water balance calculation of river network, which takes interval flow, water intake and water diversion as well as water regulation effect of multi-reservoir system into account. By calculating the natural ecological base flow of the sections where reservoir dams are sited, the conventional reservoir operation rules coupling the requirements of minimum ecological discharge are put forward. Referring to the idea of aggregation and decomposition of a large-scale system, the idea of iterative optimization of river network model of the Hanjiang River Basin is put forward. The secured ecological flow level under conventional regulation is evaluated with the flow processes in typical years. On this basis, optimal operations are conducted and the potential of secured ecological operation on multi-reservoir in the basin. The results show that reservoir conventional operation scheme coupled with the minimum discharge requirements can basically secure the ecological flow demand at the control sections with various flow processes, under the condition of independent operation in reservoirs. Reservoirs in the Hanjiang River Basin can meet the ecological flow demand at the control sections through optimal joint operation. Carrying out multi-reservoir ecological operation based on graph theory is feasible and practical.