To understand the water quality status of the main stream of the Nyang River and the main pollution factors affecting the water body, surface water samples from 10 monitoring sections along the mainstream of the Nyang River were collected during the high-water (July-September) and low-water (January-March) seasons in 2018. Water quality indexes of chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total nitrogen (TN), total phosphorus (TP), Pb and Cd were selected for analysis. Water quality of the mainstream was then evaluated by single-factor assessment, comprehensive pollution index evaluate and principal component analysis. The results of single-factor evaluations show that COD is the main factor determining the type of water quality in the Nyang River, which belongs to Class III when COD is not included in the evaluation. The results of the comprehensive pollution index method show that the water quality of the Nyang River is relatively good as a whole. To clean the water body, more than 70% of the monitoring sections need to meet the Class II water quality standard. COD and TN are the main pollution factors of the main water body of the Nyang River. The results of principal component analysis (PCA) show that the water quality of the Nyang River is mainly affected by NH₃-N, Pb, Cd, TP and COD, and that the area with the most severe pollution is the middle reaches of the river. The three methods are an organic combination of qualitative and quantitative evaluations, and the evaluation results were largely consistent between methods. Generally, the water quality in the high-water season of the Nyang River is better than that in the low-water season. However, there are many differences between the screening results of water pollution factors; therefore, a variety of important methods are used to analyse the main pollution factors of the water body.