Abstract: In water filling process, a free-flow branch tunnel shows transient flow characteristics and complicated flow conditions. In this work, we model the flow process using experimental tests on a normal scale model of a branch tunnel and 3D numerical simulations based on the standard turbulence modeling, focusing on velocity distribution, pressure distribution and flow pattern under different gate openings. By comparing test data and numerical simulations, we obtain water surface profiles at different time instants during the filling process, an optimized combination of the openings of two gates that meets the design discharge ratio, and a better tunnel shape.