Against the backdrop of global climate change, especially the escalation of ocean acidification, whether river sediments entering the sea will release elements, and to what extent such releases will occur, these unresolved questions critically impact biogeochemical cycling and ecological functioning across the world’s oceans. As a heavy metal, the excessive presence of manganese (Mn) in rivers can pose significant hazards to both ecosystems and human health. This investigation employed controlled laboratory experiments to quantify Mn releasing from Changjiang Estuary surface sediments across a pH gradient spanning 4.0–8.0 (increments: 4.0, 5.0, 6.0, 7.0, 8.0). BCR sequential extraction shows that the exchangeable fraction takes the most percentage (50%), the second is the residual fraction (30%), the third is the reducible fraction (15%) and the last one of oxidisable fraction. The rate of Mn release increased significantly with decreasing simulated seawater pH, while increased as the initial simulated seawater pH changed from 7.0 to 8.0.Release kinetics reveal that 40% of Mn element cannot be released, and the last 60% is easily to release and not easy to be released. This research indicate that enhanced oceanic acidification could mobilize substantial Mn reserves from benthic sediments, a phenomenon warranting critical attention.