Nitrous oxide (N2O), a potent greenhouse gas, is a major contributor to climate warming. Urban rivers, characterized by substantial pollutant inputs, are recognized as hotspots for N2O emissions. However, N2O emissions in rivers with weak hydrodynamics in plain areas remain unclear. This study investigated N2O emission patterns in rivers flowing in to Lake Gehu, a plain urban area in China, and analyzed functional microbial abundance using quantitative PCR (q-PCR) to explore the underlying mechanisms during both the dry and wet seasons. The results showed no significant seasonal variation in N2O emissions but highlighted pronounced spatial differences. N2O emissions decreased significantly along the flow direction and were positively correlated with the distance to the lake. For every 1 km decrease in distance from the lake, N2O emission flux decreased by 0.05 mg/(m2·d). Furthermore, N2O emission flux increased with the ratio of nitrifying to denitrifying bacteria, highlighting the critical roles of nitrification and its substrate, ammonium, in regulating N2O production. The average EF?r value in the study area was approximately 5.5 times higher than the default value in the IPCC-2019 guidelines. These findings enhance the understanding of N2O emissions from urban rivers in plain areas and provide a scientific basis for improving regional emission estimates.