Abstract: In order to achieve long-term stability and cleanliness of urban river water quality, there is an urgent need to research new methods for precise source tracing and analysis of pollutants. This paper takes the SZ River as an example, collected comprehensive data on hydrology, water quality, and meteorology. A high-resolution hydrodynamic water quality model (IWIND-LRC) was constructed, incorporating tidal effects through open boundary conditions. Using this validated model, an efficient numerical pollutant source apportionment algorithm was applied to quantitatively trace the contributions of ammonia nitrogen at the SZ River estuary. The results indicate that external sources are the primary contributors to ammonia nitrogen at the estuary, accounting for 86.06%-88.81%. Among these, the BJ River, SW River, and WT River are the most significant tributaries, contributing 20.15%-20.55%, 15.98%-16.18%, and 9.53%-9.63%, respectively. Sediment endogenous sources contribute 10.82%-11.81%. This study provides a scientific basis for water quality management in the SZ River and offers a valuable reference for pollutant source apportionment in other tidal rivers.