Water bloom is the results of long-term accumulation of nutrients in ecological eystem,and an extreme state.a result of eutrophication system after long-term evolution. The proposed method needn't define the weight of evaluation indices so that the in-ffuence of subactive factors on assessment can be minimized. Based on catastrophe theory,this paper takes water chlorophyll-a con-centration as characterization factor of blue-green algae bloom outbreak degree,total phosphoms and total nitrogen concentration as the environmental factors of potential risk evaluation indexes,and blue-green algae bloom area,extent and frequency the historical risk evaluation indexes. These panuneters can be used to establish a mufti-criteria risk evaluation index system for blue-green algae bloom outbreak. It carries out the risk evaluation combined with different regions in Lake Taihu. The results show that the risk in the western coast of Lake Zhushan,is the most serious,heavy in Meiliang Bay,moderate in southern coast of Gonghu and Large Taihu, and almost none serious in Lake Jianhu,East Lake Taihu and Lake Xuhu. In general,the degrees of blue-green algae bloom risk in Lake Taihu,from light to heavy changes from the southeast to the northwest,which is consistent with the nutrient concentration trend from low to high. The evaluation results define the possibility and the degree of the hazards of the risk of blue-green algae bloom outbreak in Lake Taihu, and provide scientific basis for the management and emergent treatment of blue-green algae bloom outbreak risk.