The article constructed the prediction and evaluation model of water environment carrying capacity of water environment capacity and ecological environment water demand based on the profound research of water environment carrying capacity systems, and formed a set of mature water environment carrying capacity research systems which is suitable for the arid area of northwest China short of water resources. Taking Weihe River in Shaanxi as an example, this paper applied system dynamics combined with analytic hierarchy process to simulate the dynamic changes of river water environmental system through the establishment of river water environmental capacity model and ecological water demand model, and forecasted the water environmental carrying capacity for different schemes from 2010 to 2020. The results showed the integrated scheme of open-source, throttle and pollution control can effectively improve the water environment carrying capacity in Weihe River basin.The constructed model has good applicability to the evaluation of water environment carrying capacity in northwest arid areas.