Abstract: A new two-step control method based on the principle of optimality is proposed for the gravity-flow in open channel system.The first step is to control all the levels (or volumes) in the channel to reach the goal levels (or volumes).The second step is to adjust the channel flow of all sections to realize the stabilized level.The optimization flow control model is built,which make the levels reach the goal levels in the shortest time.The corresponding constraints for this model,the downstream water demands,the maximum water levels,the allowable drawdown rate of water level,the allowable discharge capacity of the channel,the minimum frequent of switching on and off the pump,and so on,are constituted.The solution to the problem is proposed by using linear the program method or decomposing a large and complex system into two or three small and simple systems.The example application of this method shows that the time during which the water levels reach the target levels and keep them stable can be reduced from a few days to a few hours.