Identifying the pattern of delta morphological change under decreasing sediment flux due to dam construction is essential for sustainable management in such densely populated coastal areas. In this study, we investigated the morphological processes of the Yangtze mouth bar and prodelta based on bathymetric data on a decadal-interannual scale (1958, 1978, 1997, 2002, 2007, 2010, 2013 and 2015). We found that strong accretion (205.1 Mm³ yr⁻¹) occurred during 1958–1978, when a high sediment load (465 Mt yr⁻¹) was supplied by the Yangtze. Afterwards, the net accumulation rate decreased to 31.9 Mm³ yr⁻¹ in 1978–1997 and 114.6 Mm³ yr⁻¹ in 1997–2002 as a result of riverine sediment loads decreasing to 390 Mt yr⁻¹ and 314 Mt yr⁻¹, respectively. Surprisingly, the net accumulation rate increased to 130.8 Mm³ yr⁻¹ in 2002–2007, though the sediment load sharply decreased to 177 Mt yr⁻¹. This anomaly was attributed to the construction of training walls within the mouth bar area, which induced significant accretion in groyne-sheltered areas and nearby regions. Along with a further decrease in sediment load, the entire study area converted to net erosion of −200.4 Mm³ yr⁻¹ in 2007–2010 and −152.2 Mm³ yr⁻¹ in 2010–2013. Stronger erosion in the former period was partly caused by intensive dredging activities in the mouth bar area. The critical sediment discharge for the Yangtze mouth bar and prodelta to retain net accretion was estimated to be ca. 218 Mt yr⁻¹. If deducting the impacts of estuarine engineering projects on accretion/erosion during 1997–2010, the critical sediment discharge is adjusted to ca. 234 Mt yr⁻¹. In combination with previously reported accretion-erosion conversion elsewhere in the Yangtze Delta, we inferred that most portion of the subaqueous delta has most likely converted from net accretion to net erosion in response to fluvial sediment decline, and the mouth bar area showed the latest conversion among portions of the delta. Integrated assessment and adaptive strategies are urgently required for the Yangtze Delta to survive the coming erosional stage.