Abstract: The black water cluster(BWC)has occurred in the north part of Taihu Lake,especially bloomed in near-bank of Gong Lake and Yixing in 2007 and 2008.This paper analyzes this phenomenon using the observed data and historical data. The results indicate that the main cause of forming BWC is the combination of the high pollution load,the dead algae in water, the sludge distribution,the wind of fields,the temperature fields,and the water level in the lake area.It provides a way and basis for the study of formation mechanism and control counter measures of BWC.