Abstract: As a critical natural ecosystem supporting the survival of humans and other organisms, the water quality of rivers directly influences ecological security and sustainable development. To investigate the spatiotemporal variations in water quality of the Xiaoqing River mainstream, this study analyzed and evaluated these characteristics using the comprehensive water quality index (WQI) method based on monitoring data from 2019 to 2022. Additionally, a principal component analysis-multiple linear regression (PCA-MLR) model was employed to identify pollution sources and quantify their contribution rates in the mainstream. The results indicated that the annual average WQI of the Xiaoqing River mainstream showed an increasing trend, with the water quality grade improving from "moderate" to "good" from 2019 to 2022. No significant seasonal variation in water quality grade was observed, though the WQI during the wet season was slightly higher than in other periods. Spatially, the WQI values along the Xiaoqing River mainstream generally exhibited a pattern of midstream < upstream < downstream, with the best water quality observed in the downstream section. Poorer water quality was primarily distributed in the midstream and upstream regions, particularly in areas with a high proportion of built-up and agricultural land along the main channel. Source apportionment results revealed that pollutant sources in the Xiaoqing River mainstream were diverse, with primary contributions from industrial wastewater discharge (25.81%), agricultural activities (24.92%), and domestic sewage coupled with livestock farming (17.60%). The predominant pollution sources were generally consistent across all monitoring sections, though dominant types varied spatially, which was closely associated with land use patterns and anthropogenic activities along the river. The findings of this study can provide an important scientific basis for pollution control decision-making in the Xiaoqing River Basin.