Abstract: For water supply reservoirs in parallel,through the mutual connection of common users, it is necessary not only to determine the water supply decision according to the state characteristics of each member reservoir, but also to reasonably allocate common tasks of water supply according to the state characteristics of each member reservoir. Considering the allocation of common tasks of water supply for interbasin reservoir groups, in order to formulate scientific, effective and reasonable, balanced and equitable inter-reservoir allocation rules for common tasks of water supply, and seek optimal operational strategies for common scheduling rules of reservoirs in parallel, the way in which a reasonable balancing curves are formulated, the distribution proportion of common tasks of water supply is determined to show the optimal allocation rule in the different scheduling period of reservoirs, the mutation point and slope are used to described the segmentation of balancing curves, and changes in water storage and supply of reservoirs during flood seasons and non-flood seasons are characterized. The optimization model is established with the minimum water shortage rate and the minimum water abandonment as the objective functions to modify the allocation rules. Finally, the particle swarm optimization algorithm based on immune evolution is used to calibrate the mutation points and slopes of the decision variables of the balance curve, and finally the relatively excellent water supply rules are obtained. The reservoir groups use the downstream of the Luanhe River as an example, the research findings show that the suggested method can achieve operation goal effectively, divide common tasks of water supply reasonably and also improve computational efficiency of common tasks of water supply of reservoir groups. The reasonable balancing curves are formulated to allocate the common tasks of water supply of reservoirs in parallel. The water supply of the reservoir groups system increases by about 5% compared with the compensation control, and the amount of abandoned water decreases by about 10%, which is close to the maximum water supply, annual average abandoned water and guarantee rate obtained by dynamic planning. The distribution ratio of water supply tasks between reservoirs is close to the ratio of its beneficial reservoir capacity, the member reservoirs have larger water supply capacity and higher synchronization of storage rate of member reservoirs, and reflects the rationality and superiority of the balancing curves to a certain extent.