Abstract: The water quality of irrigation canal is crucial for agricultural cultivation, industrial production, and resident livings. In this study, a five-year water quality monitoring program was conducted in the lower Western Main Canal of the Ganfu Plain. The aims were to investigate the variations of water quality parameters, to identify and quantify the major pollution sources using principal component analysis (PCA) and the absolute principal component score-multiple linear regression (APCS-MLR) receptor model. The results show that the total phosphorus (TP) concentration ranged from 0 to 2.12 mg·L^-1 in the canal water from 2017 to 2021, with most values exceeding the Class Ⅴ standard limit of the Surface Water Environmental Quality Standards (GB 3838-2002). The total nitrogen concentration primarily ranged between 1.0 and 2.0 mg·L^-1, with some values exceeding 2.0 mg·L^-1, surpassing the Class Ⅴ standard limit. The ammonium nitrogen (NH₄⁺-N) concentration mostly ranged between 0.01 and 0.1 mg·L^-1, whereas the nitrate nitrogen (NO₃^--N) concentration changed between 0.4 and 1.5 mg·L^-1. PCA analysis revealed that canal water quality was mainly affected by meteorological factors, agricultural activities, and water-sediment interactions in the wet season, while it was associated with industrial wastewater, domestic sewage, and livestock farming in the dry season. The APCS-MLR model further quantified the pollution source contributions: agricultural cultivation and livestock farming contributed 45.3% and 32.7% to TP, respectively, meteorological factors and unknown sources accounted for 38.5% and 28.9% of TN and NO₃^--N, respectively, and domestic sewage, surface runoff, and unknown sources contributed 41.2%, 25.6%, and 33.2% to NH₄⁺-N; livestock farming contributed 56.8%, 48.3%, and 52.4% to TP, TN, and NO₃^--N, respectively, while domestic sewage and livestock farming accounted for 62.1% and 37.9% of NH₄⁺-N, respectively. The findings enhance the understanding of nitrogen and phosphorus pollutant cycling in irrigation canals and provide a scientific basis for water quality management and protection.