A complete joint utilization for a multi-reservoir system should consider two operating stages: one is to investigate how much water should be released to meet the target demand, and the other is to decide the desirable end-of-period storage for each single reservoir. To simulate the joint operating procedure in a large scale water resource system, a simulation model, named WRASIM-M, was developed by this study based on Network Flow Programming. Two primary advantages from utilizing WRASIM-M are the comprehensive water allocation concept and the computational efficiency. WRASIM-M could be regarded as a simulation tool to help establishing joint operating rules, especially suitable for a large scale and complex multi-reservoir system. The results of case study validated the simulating function of WRASIM-M. Copyright ASCE 2006.