Abstract: In order to explore the disaster-causing impacts of different combinations of tributary floods on the mainstream area, a one- and two- dimensional coupled hydrodynamic model was constructed and validated. The coupled model was applied to model an ideal Y-shaped basin and the Sanjiangkou area in Ningbo. Simulations of tributary flood processes with different peak time lags were conducted, and five indicators-mainstream peak flow, inundation volume, inundation area, maximum inundation depth, and maximum overflow distance were selected to reflect the disaster situation caused by floods in the mainstream area. The results indicate the following:(1)The coupled model constructed in this study can effectively reflect the exchange of water between one-dimensional river channels and two-dimensional land surfaces. (2)In the ideal Y-shaped basin, as the peak time lag of tributary floods increases, the disaster impact on the mainstream area significantly reduces. The five indicators selected in this study decrease by 1.52% to 22.34%, 5.70% to 75.48%, 8.49% to 97.60%, 1.88% to 36.45%, and 2.03% to 37.56%, respectively. (3)As the peak time lag between Yuyao River and Fenghua River floods increases, the impact of floods on the Yong River area significantly decreases. The five indicators selected in this study decrease by 10.03% to 21.47%, 52.28% to 94.16%, 56.05% to 97.55%, 11.69% to 28.46%, and 29.21% to 76.64%, respectively. This study can provide certain references for flood prevention work in plain river network areas.