Assessment of water resources carrying capacity is a complicated system with multi-objectives. The difficult problem is how to establish a rational mathematic model for water resources carrying capacity assessment system. Since the assessment standard actually is not a point but an interval, a defect exists in the traditional multi-objective decision-ideal point method of taking the assessment standard as an ideal point. In order to overcome the defect, a new method--Multi-Objective Decision-Making Ideal Interval Model (MODMIIM) is suggested, its constitutional principle and method are given for assessing water resources carrying capacity, and its detailed step is developed. In addition, MODMIIM has been used to assess water resources carrying capacity in the Huaihe River Basin. The result shows that the water resources carrying capacity in the Huaihe River Basin is rather low. Comparing with the multi-objective decision-ideal point method, MODMIIM has the features of good adaptation with strong maneuverability and the stable and rational calculated results. As a good and simple assessment method, MODMIIM can be used for comprehensive assessment of water resources.