The sustainable development of water resources has been an important issue particularly. Therefore, the long-term water supply plan in a basin scale needs to be evaluated with the consideration of future uncertainties. However, the complexity of the model and future uncertainties cause the difficulty to analyze the water supply model under the various scenarios. System dynamics approach is used to develop a water supply system in the Han River basin, South Korea. The developed model is calibrated by the water supply data and the basin condition is evaluated with respect to the water availability. The future water supply scenarios are developed and the water excess and shortage are evaluated under the various water supply scenarios. The main drag forces to cause this water shortage in the basin are the increasing population in urban areas and future climate change. The system dynamics approach helps the decision maker and stakeholders to understand the model structures, scenarios, and the corresponding results. The developed model is a useful tool to evaluate the long-term water supply plan when the policy maker develops a city development plan including water supply and achieve consensus.