By taking the Yellow River Delta as a prototype, an idealized model featuring typical river mouth bar topography was constructed using Delft3D. The model was used to simulate and analyze the influence of vegetation on the topographic evolution of the river mouth bar in the Yellow River Delta under water and sediment regulation. The results show that with vegetation, the sediment deposition volume of the river mouth bar after the water and sediment regulation is reduced by 63.7% compared to the scenario without vegetation, which contrasts with the general understanding that vegetation promotes sediment trapping and accumulation. The flow resistance effect of the vegetation on the river mouth bar alters the flow partitioning ratio between the bifurcated channels and the river mouth bar, reducing the flow discharge over the bar by 70%-85%. This redirects sediment-laden flows primarily through the river channel to the sea, leading to a decrease in local sediment supply and trapping on the river mouth bar. Furthermore, the geomorphic effects of vegetation differ significantly between the water-regulation period and the sediment-regulation period. In the absence of vegetation, vertical accretion and lateral expansion of the river mouth bar mainly occur during the sediment-regulation period, characterized by low discharge and high sediment concentration, with almost no deposition during the water-regulation period. In contrast, when vegetation is present, significant vertical accretion of the river mouth bar occurs during the water-regulation period, while its lateral expansion mainly takes place during the sediment-regulation period and is confined to the direction of sediment supply.