Based on water quality information from 6 stations in the upper-reach lakes of Lake Nansi which was sampled from a total of 42 odd months during 2008 and 2014, choosing dissolved oxygen, COD, COD_Mn, BOD₅, ammonia nitrogen, total phosphorus and fluoride as the indexes of water quality evaluation. The water quality index normalization was used to improve the method, and the principal component analysis was used to evaluate water quality comprehensively for the data from the six monitoring stations. According to the obtained comprehensive score of the monthly sequence, the trend of water quality status was analyzed. Compared with the general condition of water quality during 2000 and 2007, it is found that water quality was significantly better during 2008 and 2014. The evaluation results showed that the water quality at any monitoring station had obvious changes at different time, and had different changes at any time sequence on different water quality monitoring stations. Besides, the main influenced indicators of water quality in the upper-reach lakes of Lake Nansi were COD, COD_Mn and BOD₅. According to the evaluation results and the analyzed changes in the water quality, we found that the river quality into the lake and lake sediments were the two important factors influencing on the water quality.