Abstract: The Xin’an Reservoir is a strategic water source in the Yangtze River Delta region of China, and it is also a rare high-quality water resource. Analysis of monitoring data reveals that specific water quality parameters in this region cannot reach the drinking water standard. To identify pollution sources associated with various water quality parameters in the study area, this study analyzed 13 189 monthly data of 11 water quality parameters from 8 monitoring stations between 2002 and 2020. By using the Absolute Principal Component Scores-Multiple Linear Regression（APCS-MLR）method, a quantitative analysis of the pollution sources in different water periods was carried out. The results demonstrated that total nitrogen（TN）was the primary pollutant exceeding the standard in the upper reaches of Xin’an River basin, with agricultural non-point sources and industrial emissions identified as dominant contributors. During the wet season and the dry season, TN was mainly affected by agricultural non-point sources and industrial sources, with contribution rates of 28.8% and 35.8% respectively. While during the normal season, TN mainly came from industrial sources, with a contribution rate of 49.7%. Similar to TN, other pollutants also have different sources in different water periods. Therefore, different measures should be adopted to improve the water environment in different periods. This study provides critical insights for pollution source identification and watershed management, offering scientific support for developing targeted water quality protection strategies in the study area.

Key words: : Xin’an River Basin;, water season, APCS-MLR, source apportionment of pollutants