Abstract: The factors offecting the water out let evolvement of Cao'e River are set forth:there are some nature and artificial factors of water outlet evolvement of Cao'e River,including mainly the deflection force of earth rotation,the runoff volume,the sediment volume,the human act ivities and such like.The water outlet of Cao'e River swings frequently,which is controlled to a great extent by the change of Q iantang River s main channel and its south channel.On the condition of flood control and reclamation of the tidal flats along two banks,the optimum alignment of the water outlet of Cao'e River is northeast.The dredging principle is put forward "to guide it along its course of development and bring the river under control by means of reclamation of the tidal flats".Blocking the south channel can stabilize the alignment of Qiantang River's River to the northeastwards,which at same time can enlarge the reclamation area of the tidal flats.