For natural water,method of water quality evaluation based on improved fuzzy matter-element evaluation method is presented.Two important parts are improved,the weights determining and fuzzy membership functions.The coefficient of variation of each indicator is used to determine the weight instead of traditional calculating superscales method.On the other hand,fuzzy matter-elements are constructed,and normal membership degrees are used instead of traditional trapezoidal ones.The composite fuzzy matter-elements with associated coefficient are constructed through associated transformation.The levels of natural water quality are determined according to the principle of maximum correlation.The improved fuzzy matter-element evaluation method is applied to evaluate water quality of the Luokou mainstream estuary at the first ten weeks in 2011 with the coefficient of variation method determining the weights.Water quality of Luokou mainstream estuary is dropping from level Ⅰ to level Ⅱ.The results of the improved evaluation method are basically the same as the official water quality.The variation coefficient method can reduce the workload,and overcome the adverse effects from abnormal values,compared with the traditional calculating superscales method.The results of improved fuzzy matterelement evaluation method are more credible than the ones of the traditional evaluation method.The improved evaluation method can use information of monitoring data more scientifically and comprehensively,and broaden a new evaluation method for water quality assessment.