Ecological system, especially wetland is suffering form gradually degeneration in recent years, but due to the long length of classification standard span, the current wetland ecological water demand and ecological water level could not reflect the actual continuity characteristics of ecosystem changes. Therefore, it is possible to evaluate wetland ecosystem health condition and establish relationship between membership degrees and water level of ecosystem health by taking advantages of fuzzy description using fuzzy set theory. In this way, membership function of ecological water level is established to characterize continuity and rate ecosystem health with water level, which is applied in Baiyangdian Wetland.