Beijing，as the capital of China，is also a city without enough water. In this study，a water resources supply and demand simulation model of Beijing is constructed. The model is composed by two parts：demand and supply. Water demands include domestic water，environmental water，industrial water and agricultural water. Water supply consist of surface water，groundwater，reclaimed water，“South to North” water and emergency water. The simulation model designed variables based on reclaimed water changes，south-to-north water diversion，permanent population and water consumption per capita. Predicted water consumption/ supply have been compared with preset target. According to the results，five executable suggestions are given from different aspects.