This paper analyzes the mixing zone of a vertical discharge of sewage into a natural tidal river with strong tidal currents. The paper presents a numerical model, which combines 1–0 and 2-D models to compute the mixing zone for the Sibao Segment of the Qiantang River. The simple 1–0 model was used to model the flow for the entire river using field data as the boundary conditions. The complete depth-averaged turbulence model was used for the 2-D computation. The calculated results agree well with the field observations. The analysis provides a practical method for the computation of mixing zones in tidal rivers.