From May to September each year, a large amount of farmland irrigation water is discharged into Chagan Lake. In order to evaluate the impact of farmland withdrawal water on the water quality of Chagan Lake, based on the water quality monitoring data of Chagan Lake, the EUTRO module of WASP model is selected to analyze the water quality of Chagan Lake from farmland irrigation water. The results showed that during the period of water withdrawing from the farmland, the water quality of the region affected by the water of Huolin River is worse than the average level of Chagan Lake water quality, and the water quality of the direct farmland withdrawal water affected area of Qianguo irrigation area and Xinmiaopao is worse than that of the main lake region and Mayingpao, showing that farmland withdrawal water has negative effect on the water quality of Chagan Lake. The author of this paper thinks that TN of Chagan Lake is in the Ⅴ-level water quality standard, while TP in Ⅳ-level, NH₃-N in Ⅱ-level, and DO in I-level from 2017 to 2019.