Seeing that the observed vertical distribution of pollutant is not uniform, in this article a three-dimensional numerical model on the tidal current and the COD dispersion in the Haihe River mouth and Bohai Bay was develped based on Leendertse (1979). The horizontal and vertical distribution of tidal current and COD concentration computed on this model are all in agreement with observations. By this model the maximum allowable load of pollutant for that area (so called environmental capacity) was discussed. It was pointed out that, the yearly total of pollutant discharged into that area was never beyond the limit up to now, but there did occur pollution near-shore at the upper layer sometimes, the reason is that the pollutant discharge speed was extremely uneven in the course of a year, and that a considerable amount of wastewater suddenly came from a few sites in rainy season.