Abstract: Urban hydraulic structures, mostly built in the small tributary streams of the city, play an important role in the city flood control and drought relief and urban water environment beautification. Adopting VOF method to deal with the free surface, RNG turbulence model was used to simulate the flow characteristics of tributary confluence. The water flow characteristics such as water level, velocity, turbulent energy dissipation rate and bed shear force under different confluence forms of small tributaries with large-scale river were analyzed. The results show that the main factors affecting the flow characteristics are the tributary flow and the upstream trend of tributary river. The research results can provide reference for the design and construction of the hydraulic structures built on the urban small tributaries in confluence area.