Water exchange is an important hydrodynamic character of sea bays, and it is the basis for the study of the environmental capacity of sea bays. In this paper, a relation matrix is set up to describe the interaction among different areas of a sea bay, and to predict the water quality of those areas. The relation matrix is calculated based on the numerical results from a water quality model. This method is applied to the study of water exchange and the prediction of water quality of the Bohai Sea. The Bohai Sea is divided into five areas, and the effect of seasonal wind is taken into consideration. The results show a） the relation matrix can be used to study the water exchange among different areas and predict water quality of different areas at the respective characteristic time, b） the reduction of pollutant is dependent on both water exchange and initial distribution of the pollutant, and c） the half-life time of the pollutant is longer than the half-exchange time of the sea water.
