In order to evaluate water quality of lake, a new assessment model is proposed. The new evaluation model is on basis of variable fuzzy sets that can consider the good and bad character of indexes, considering the fuzzy feature in assessment. Apply the model into water quality assessment in West Lake, Qinghai Lake, Chao Lake and Dianchi Lake, and sort samples by lake. Results show that new assessment methods are feasible and can effectively evaluate water quality of level.