Based on the analysis of water demand predication and water resources constraint, this paper investigated the quantification indicator of water resources constraint. It put forward that the forecast of urban water demand should be combined with the water resources constraint.The coefficient of water resources constraint was introduced to the predication of water demand. The paper set up the prediction model of urber water demand under water resources constraint by using BP network.Taking Zhengzhou for examble,it applied the model to predict the water demand and analyzed the result of predicttion. The results indicated that the model has certain practical value and can provide scientific basis for the plan of zone development,the scheme of water supply project and water saving.