Abstract: Taking Nanwan Reservoir in the upper reaches of Huaihe River basin as the research object， an enhanced single-factor water quality identification index method was presented， the time variation characteristics of its water quality were monitored and evaluated. The results showed that： （1） The presented method could effectively demonstrate whether the water quality meets the functional zone standards， as well as highlight the specific values by which the water quality exceeds or falls short of these standards， showcasing its superiority in accuracy compared to conventional methods.（2）Most of the water quality indicators in most months of 2018—2019 meet the Class Ⅲ water standard， and only some months of TN total nitrogen exceed the standard. The total phosphorus term was also close to the standard critical value. （3） The eutrophication degree of TP of Nanwan Reservoir was evaluated by comprehensive trophic state index method， and the results showed that the whole lake was in the stage of poor eutrophication. （4） The characteristics of time variation of water quality were as follows： dissolved oxygen DO， BOD₅ and TN were the indicators for slight improvement between years； COD_Mn， TP， anionic surfactant， chlorophyll a， Cr⁶⁺ were the indicators with little change between years. The indicators of interannual variation were transparency and fecal coliform. The seasonal variation characteristics were as follows： DO value of dissolved oxygen showed a decreasing trend from winter， spring， summer and autumn， TN and TP contents were higher in summer and autumn， and petroleum substances content was higher in August of petroleum summer.