The paper analyzed the monitored data of north Dagang reservoir in May、July and September of 2013 by use of fuzzy comprehensive evaluation method of entropy weight, diversity index and dominant specie of plankton. The result showed that the water quality of North Dagang reservoir in May,July and September of 2013 belonged to Ⅳdegree water, and which of diversity index of plankton was high, but there were some pollution tolerant species in dominant specie of plankton. Both the result of water quality assessment and plankton showed that the water is in the state of eutrophication.At last it proposed some improvement suggestions based on the research result.