Based on the idea of Lagrangian time-average residual current, a coastal water circulation model which can describe the circulation in a coastal water-shelf s ea system where tidal currents and some quasi-steady flows, such as the Kuroshi o in the East China Sea, are of the same order and dominant in the observable fl ow field, is developed. In this paper, the summer circulations in the Bohai, Yel low and East China Seas are diagnostically simulated by a sigma coordinate semi -implicit numerical scheme usi...