Abstract: The confluence of tributaries often leads to the increase of local transverse flow in main channels, which affects the navigation safety.In order to study the influence of excessive transverse flow on the channel scale of the main stream, one two-dimensional flow mathematical model and a ship maneuvering motion model were developed independently by Fortran language.After the rationality and reliability of the model were proved, the influence of excessive transverse flow on the transverse drift velocity and drift angle of ship navigation was studied by simulating the ship motion under the action of excessive transverse flow, the safety range of excessive transverse flow and the quantitative widening value of the channel were proposed.The practical application showed that under the traditional channel scale design, when the excessive transverse flow at the estuary is contained in a limited range, the navigation safety can be realized by adjusting the rudder angle; while the excessive transverse flow range is large, it is difficult to ensure the safe navigation of ships, so the river channel is required to widen.The determination method of channel widening proposed in this paper is basically reasonable, which has certain reference values for ship maneuvering motion and channel design.