Begional river water quality is a comprehensive reflection of environmental characteristics and human socio-economic activities. Though it is very complicated and quite fuzzy, yet its space distribution and composition obey some rules. In this paper, fuzzy cluster analysis is used to distinguish river water quality in Beijing-Tianjin-Tangshan area. Then a dynamic fuzzy cluster tree was drawn up, by fuzzy matrix multiple calculation. After determining a proper threshold, i.e. value λ, according to actual condition, an objective classification of river water quality was obtained. The result was compared with those obtained by systematic classification, the former reveals interdependency between different environmental factors.