Abstract: Based on the flow characteristics of a tidal river, a quasi-dynamic water quality reciprocation model is pressented. The model is solved by using a similar Pearson Type Ⅲ distribution. A method for evaluating the model parameters is given. The model is verified against the amonia-nitrogen concentration data observed in the course of Huangpu River. From the satisfactory results, it appears that the model would be applicable to analysing the distribution and changes of decomposable organic pollutant concentrations.