River pollution is a significant problem within the urbanization process in China. Nitrogen is one of the most important pollutants in rivers, and the nitrogen purification capacity of rivers can be affected by their sinuous morphology. In this study, a set of sandy circulating water test models was constructed, consisting of four river channel simulation models with sinuosities of 1.0, 1.4, 1.8, and 2.2. Each model was then infused with the same concentration of nitrogen-polluted water, which circulated for 52 h. The nitrogen reduction processes of rivers with different sinuosities were studied through water quality monitoring. The positive correlation between river sinuosity and nitrogen purification capacity was verified in physical laboratory test models. The effect of sinuosity on the spatiotemporal distribution of total nitrogen in pore water was confirmed. Additionally, the near-shore substrate was more involved in the process of river self-purification than the far-shore substrate. The concave bank of the sinuous rivers was more prone to pollutant accumulation and had a higher purification capacity than the convex bank. After the polluted water entered the sinuous channel systems, pollutant concentration differed within the convex bank between the more polluted upstream section and the less polluted downstream section. This study lays a foundation for studying the mechanism by which river sinuosity influences self-purification capacity.