Black patch events caused by cyanobacterial blooms in Lake Taihu were very special ecological phenomenon and crucial important ecological disasters. Clarifying the cause of formation and mechanism of black water event is very useful for the risk forecasting and prevention. In this paper, occurrence of black patch events during 2009 and 2017 and their relationship with the environmental factors in Lake Taihu were analyzed. Based on the daily survey undertaken by Jiangsu Bureau of Hydrology and Water Resource, 61 black water events which larger than 0.01 km² were reported during 2009 and 2017, with 3 to 17 times per year. More than half of these black patch events happened at the northwestern coastal region of Lake Taihu, where there is the most polluted input. Continuely hot weather, low air pressure and low wind speed conditions for five days benefited the occurrence of the black patch events. Southeast wind direction also made the bloom materials moving to northwest coastal region and product black patch, which bring lots of pollution from that region. Accumulation of massive bloom material supplied the substance condition for the black patch events. In addition, nutrient loading of the former year was also related to the intensity of black patch event in Lake Taihu. The research indicated that Lake Taihu has high risk of black patch event, although plenty of restoration project has been demonstrated in the past 10 years. Warmer climate and high nutrient background were the major factors delayed the tremendous efforts in the past 10 years. Therefore, monitoring and forecasting are still very necessary for the control and management of risk from cyanobacterial blooms in Lake Taihu.