Abstract: When the tributary discharge in the reservoir channel is small, the water and pollutants from the tributary are retained in the tributary confluence due to the obstructing effect of the main stream, and in serious cases, it is easy to burst large areas of algal bloom in tributaries. In this paper, the hydraulic characteristics of tributary confluence under weak mixing conditions are investigated by using numerical simulations combined with flume experiments. The RNG k-ε turbulent model is used to close the control equations, and a three-dimensional numerical simulation model of the open channel intersection flow is established, and the simulation results such as vertical flow velocity distribution and velocity vector diagram are in great agreement with the open channel flume experiments data. Based on the numerical simulation results of 4 different convergence ratios and 3 different junction angles conditions, the flow structure partitioning of the tributary confluence under weak mixing conditions is proposed, and the effects of different junction angles and convergence ratios on the key hydraulic characteristics of the tributary confluence are analyzed. The study found that, under the weak mixing conditions, the right bank of the tributary confluence generates backflow to form a backwater zone, and the water of the left bank is squeezed to form an acceleration zone, and the peak backflow velocity occurs near the right bank wall; the length of the backwater zone under different junction angle conditions is 150°＞30°＞90°, and the length of the backwater zone gradually decreases with the increase in the convergence ratio until the backwater zone disappears; and the distance between the maximum width of the backwater zone and the confluence is inversely proportional to the convergence ratio; both the high turbulent kinetic energy region size and value at the tributary confluence are inversely proportional to the convergence ratio and positively proportional to the junction angle, and the influence of the junction angle on the turbulent kinetic energy is greater than that of the convergence ratio.