Abstract: Climate change and human activities are the two most important driving factors influencing the land-surface hydrological cycle and water supply and demand balance. A comprehensive and systematic method is used to build a water demand prediction model for Beijing. This model has taken many factors and their mutual relations into consideration and could calculate the water demand of different sectors. The results show that:① the total water demand will increase at least 15.1% (up to 33.8%) from 2019 to 2030 and the corresponding water shortage ranges from 0.394 billion to 1.922 billion m3;② climate variation is likely to affect Beijing's water balance to a large extent, and the inter-basin water transfer project and water conservation technology play important roles in alleviating water shortage in the future;③ the proposed system dynamics model is applicable to other mega cities due to its generality in water demand calculation and applicability in scenario design and analysis procedures.