In order to ensure the navigation safety of bridge areas during bridge construction，this paper analyzes the influence of large steel cofferdam setting on waterway flow conditions. Taking the steel cofferdam setting of a bridge in Chongqing as an example，based on the fluid mass conservation equation and Reynolds time-mean equation，and the Fluent finite element software，the second-order upwind algorithm and Green gradient method are used to numerically simulate the bridge axis direction and cross-sectional velocity perpendicular to bridge axis in the steel cofferdam setting river section. Through the comparison and analysis of the change of flow velocity of each section before and after steel cofferdam setting，we obtain the influence of steel cofferdam setting on the flow condition of the channel. The research results indicate that：1）In the direction perpendicular to the bridge axis，the steel cofferdam front-end along the flow direction is about 0.4 Bs~ 3.3 Bs（Bs for the width of the steel cofferdam）velocity increment is most obvious. 2）In the direction of the bridge axis，the flow velocity in section A of the bridge increases sharply，and the turbulence caused by the setting of the steel cofferdam has an impact width of about 3.7 Bs. 3）A vortex street is formed at the tail of the steel cofferdam，which is about 1.3 times the length of the steel cofferdam and 1.5 times the width of the steel cofferdam. 4）The variation characteristics of waterway flow conditions during the steel cofferdam setting are of great significance for the maintenance of waterway navigation safety.