In this study, a three-dimensional model is developed to investigate sediment transport under tidal bores in the Qiantang estuary. The sediment transport is modelled by the advection and diffusion equation, and the exchange of sediment between the riverbed and the flow is determined by a net flux. A new formula of sediment carrying capacity based on recent field data is used and the settling velocity is resolved with an empirical formula accounting for the flocculation of fine sediment. Skill assessment is employed to show the model performance and relatively high predictive skills are demonstrated. The significant rise of sediment concentration under tidal bores is accurately reproduced. Sediment concentration with the arrival of tidal bores increases more than 10 times, and the depth-varying characteristic of suspended sediment is captured. It is concluded that the amount of sediment transport under tidal bores amplifies an order of magnitude, and the reverse sediment transport pattern is documented.