In order to study the flow structure in a spur dike field of the North Passage in the Yangtze Estuary, a two-dimensional large-scale coupling model for tidal current and water quality was established using the finite volume method(FVM)and unstructured grids. Based on model verification, the flow structure and pollutant diffusion in the spur dike field of the North Passage during flood tides and ebb tides were studied. The following conclusions are drawn: the high tide level in the north spur dike field in the middle-upper reach of the North Passage is significantly higher than that in the main channel, while the high tide level in the lower reach of the North Passage is close to that in the main channel; the high tide level in the south spur dike field is mostly higher than that in the main channel, and the low tide levels in the north and south spur dike fields are close to that in the main channel; the rate of pollutant diffusion in the south spur dike field is higher than that in the north spur dike field, with a relatively high rate of diffusion during ebb tides; the rate of pollutant diffusion in the spur dike field is higher during spring tides than during neap tides, with a relatively high rate of pollutant diffusion outside the spur dike field in contrast to that inside the spur dike field; after release of pollutants in the spur dike field, the pollutants are transported within the spur dike field and the main channel following flood tides and ebb tides, and lesser amounts enter into the waterway in a short period.