In order to explore the characteristics of the algae density and its water environment factors in Lake Nansi under the context of water transfer, non-parametric statistical methods are used to comprehensively analyze the inter-annual and intra-annual distribution trends of algae density and its key water environment factors (total nitrogen(TN), total phosphorus(TP), ammonia nitrogen, chlorophyll-a, BOD₅, COD_Mn). The results show the annual average of comprehensive pollution index of water quality fluctuates in the range of 0.759-0.945, 0.719-0.926 in upper and lower lakes from 2010 to 2020, but the TN in Lake Nansi still exceeds the standard, which was mainly related to the high input of nitrogen into the lake. TP, ammonia nitrogen, chlorophyll-a and algae density had a sharp turning during 2013-2015 because of drought and the early operation of the South-to-North Water Transfer Project. The water quality indicators all increased afterward. The east route of the South-to-North Water Transfer Project has a strong impact on the intra-annual fluctuation of algae density. The peak value occurs in October when the algae density was 520.0×10⁴ cells/L in upper lake in 2016. Algae density was not only significantly related to ratio of nitrogen to phosphorus, TP, COD_Mn, chorophyll-a, but also closely related to rainfall and water temperature. In the process of environmental protection and management of Lake Nansi, the supervision of key water quality needs to be strengthened. The management of water diversion and ecological crisis should be improved.