For a large hydro power plant with complex hydraulic system， its optimal operation is a complex nonlinear and combination optimization problem. When upstream water level and load level is constant， it is difficult to determine the optimal unit commitment， calculate the allocation of optimal unit power and water consumption in the process of actual operation. A model taking into account all kinds of nonlinear properties and complexly coupled relationships is set up. Optimal conditions and general equally incremental rates about water consumption are derived. A corresponding fast algorithm is proposed and can be used either online or off-line. A Program based on the proposed algorithm is developed and applied in real system. The results show the correctness and effectiveness of the algorithm.