Based on the monthly water quality and quantity synchronous monitoring data of Yangcheng West Lake from 2007 to 2019, the spatiotemporal variation characteristics of pollutant flux into Yangcheng West Lake and the retention rate of pollutant flux in the lake were quantitatively analyzed. The results show that the pollutant flux into Yangcheng West Lake is unevenly distributed throughout the year, and it is more easily affected by water volume when the water volume difference is large. Over the years, the pollutant fluxes into the lake showed an overall downward trend, with the largest to smallest reduction rates observed for ammonia nitrogen, total phosphorus, total nitrogen, and permanganate index. Among the eight main inflow rivers, the contribution rate of pollutants entering the lake in the Baidang River was the largest, followed by the Litang River and Jiejing River. In spatial distribution, the pollutant flux into the lake was the largest in the southern part of Yangcheng West Lake, followed by the northern and central parts. In 2019, the retention rates of pollutants in Yangcheng West Lake ranked from highest to lowest as ammonia nitrogen, total phosphorus, total nitrogen and permanganate index, and more than half of the pollutants entering the lake continued to flow into the middle lake. It is suggested to focus on the main pollutant inflow areas of the Baidang and Litang rivers, and strengthen the pollution source control and comprehensive treatment in surrounding areas.