The Changjiang River Estuary and the adjacent East China Sea is one of the world’s largest coastal carbon sinks, with a steadily increasing sink capacity over recent decades. However, the potential changes in its carbon sink and control mechanisms at decadal scales under climate change remain unclear. This study, based on 27 years (1998–2024) of continuous satellite remote sensing data, investigates the spatiotemporal distribution and long-term evolution of this coastal carbon sink. The results reveal a typical carbon sink with a capacity of −5.23 ± 3.73 mmol m⁻² d⁻¹ and significant seasonal variation. High-frequency remote sensing data reduces uncertainty compared to traditional shipborne observations. Over the past 27 years, the air–sea CO₂ flux increased at a rate of 0.24 mmol m⁻² d⁻¹ yr⁻¹, with a five-fold enhancement in carbon sink capacity. However, after atmospheric pCO₂ exceeded 400 μatm in 2014, the rate of increase slowed, indicating stabilization. Control mechanism analysis shows that biogeochemical processes have been persistently active, while over the past decade the influence of Changjiang discharge on seawater pCO₂ increased by 50%, shifting the system from primarily physical dilution to enhanced biogeochemical regulation. The findings provide insights into the evolution and management of coastal carbon cycles under climate change.