Abstract: Riverbed material excavating field may make changes in riverbed topography,which would lead to greate influence on navigable conditions. The velocity change may lead to the stop of ship sailing; the velocity direction change may give some impacts on the bank slope stability. In this article, using the fluid calculation software SMS(The Surface-Water Modeling System, created by USACE-WES and Brigham Young University, and widely used in the tidal estuary,river water flow simulation), based on the Navier-Stokes equations, a mathematical model adopting the weighted residual Galerkin finite element method is used for solving the problems. This module is to calculate unsteady flow and constant flow under the condition of water level and two-dimensional velocity universal module. The mathematical model is applied to the Ganlanba hydroelectric project which is one of the dams located on the Lancang River. Comparison analyses show that the mathematical model is in good agreement with experimental data. By simulating navigation flow conditions with presence or absence of the material excavating fields in the upstream and downstream riverbed, an analysis is made of impacts caused by riverbed material excavating field on entrance area navigation conditions. The analysis results also show that the riverbed material excavating fields have influences on navigation flow conditions of the entrance area of the approach channel, but less influences on the upstream flow patterns, and the downstream influence is more obvious. For example, the mainstream flow of the upstream does not change but the cross-section velocity is slightly lower; when the mainstream flow of the downstream changes, the mainstream flow is away from the entrance area of the approach channel, and the riverbed material excavating field can effectively reduce the oblique flow which is flowing along the entrance area of the approach channel, thus improving the flow conditions of the entrance area of the approach channel, which is favourable for the navigation. The simulation results show that the riverbed material excavating field for Ganlanba hydroelectric project is beneficial to the navigation. The research can provide a key technical support for engineering design.