Abstract: After the reservoir is impounded, the increase of water depth changes the hydrodynamic conditions of the river channel, the backfilling of the incoming sediment from main stream to the tributaries resulted the sedimentation of tributary mouth, affecting the effective storage capacity of the reservoir and the safety of navigation. In this paper, taking Xiluodu Reservoir as an example, the characteristics of water and sediment transport in the main stream and tributaries was studied, and the formation mechanism of barrier sandbar at the tributary mouth was analyzed based on the measured data and the three-dimensional mathematical model. The results is indicated that: the deposited sediment of tributaries mainly comes from the upstream incoming sediment and the backfilling of the main stream sediment, and the upstream incoming sediment is the main source of the sedimentation of tributaries; the main stream backflows into tributaries near the tributary mouth, and a convective and circulation zone are formed, where the sediment is easy to be deposited. The slope of Xixi River and Niulan River is slow, and the risk of the barrier sandbar formation is large, while the slope of Meigu River is large, and the risk of the barrier sandbar formation is small; for Xixi River, Niulan River and Meigu River, the distance of main stream backfilling into the tributary increases with the increase of discharge, and the influence of water level on the distance of main stream backfilling into the tributary is not obvious when the Xiluodu Reservoir is operating at high water level. In Jinyang River and Xisujiao River, the distance of main flow backfilling into the tributary is basically unchanged with the increase of discharge, and the distance of main stream backfilling into the tributary increases with the increase of water level. The research results provide a basis for controlling the risk of barrier sandbar formation at the tributary mouth of reservoir under the new condition of water and sediment.