The assessment to vulnerability of the water supply system needs several modeling tools and cases to simulate. A decision-support tool which integrates the common procedures of impact assessment of climate change, downscaling, weather generation, hydrological model, and interface for linking system dynamics model is proposed here. In this study, the impact of climatic change and growing water demand to the water supply system in Touchien river basin in Taiwan was derived. The vulnerability for the current water supply system was estimated for present and future conditions. The result demonstrated that the water supply system could meet the water demand in Touchien river basin but might be subjected to serious water shortage due to climatic change and expanded water demand. Results are expected to give the authorized government sectors a hint for instituting water policy and implementing allocation measures for irrigational water.