Abstract: The multiple objective decision making-ideal point method,which is applied in water environmental quality assessment,is discussed in this paper.Since the assessment standard actually is not a point but an interval,a defect exists in the traditional multiple objective decision making-ideal point method which deals with the assessment standard by using ideal point.In order to overcome the defect,a new method,multiple objective decision making-ideal interval method (MODMIIIM), is suggested and its constitutional principle and method are given for assessing the water quality.The result shows that the MODMIIM is easier and more available than the projection pursuit model and the neural network method,more rational than the traditional ideal point method.It is a superior assessment method for various comprehensive assessment problems about water environmental quality.