Taking the water demand data from 1990to 2010of Yan''an City of Shaanxi Province, this paper analyzes the main factors that influences the water resource quantity based on the principle component analysis method. The results show that GDP, rainfall, residents living water consumption and ecological environment water consumption are the primary indexes that affact the water demand. Taking the main indexes as input samples, the paper set up forcast model of water demand. The simulation results are consistent with the actual value, and use the model to predict water demand in 2015.