This paper studies the practicability and rationality of the application of grey system theory to the assessment of water quality of lakes and proposes the multi-target grey situation decision making method by using grey decision theory.Here, the water sample is taken as an event, the grade of water quality as a strategy, then a situation is set up.By using effect measure to unify the indexes with the grey situation decision making and determining the optimum situation according to the eigenvalues of grade variables, a new method is presented for evaluating eutrophication in lakes.The assessment result of nine lakes' eurtophication shows that it's practicable to assess eutrophication of lakes by using I his method.