In this paper,the fuzzy set theory and model of assessment for lake (reservoir) eutrophication are established on the basis of the fuzzy characteristics of eutrophication. Furthermore, the paper proposed firstly the concept and derived method of comprehensive weight matrix of eutrophication indices. The model is also applied to assessing the eutrophication situations of 12 lakes (reservoirs) in China and the results are correspondent to the facts. Therefore,the theory and model provide not only scientific basis for the treatment and reservation of lake (reservoir) environment but also a new method for the study on assessment for lake (reservoir) eutrophication. The theory and model proposed in the paper are also adaptable to other environmental fields concerning assessment theoretically.