Abstract: Irradiance and CO₂ are both important environmental factors affecting the biological carbon sequestration of marine phytoplankton. In order to study the response of phytoplankton assemblages to ocean acidification (OA) and irradiance changes in the "turbid zone" area adjacent to the south of the Yangtze River Estuary, a shipboard incubation experiment was conducted in May 2020. The results show that the concentrations of total chlorophyll a (Chl a), particulate organic carbon (POC) and particulate organic nitrogen (PON) were increased more significantly by increasing irradiance in the range of 350–1 200 µmol·photons·m⁻²·s⁻¹ than OA. One hundred and one Pa of pCO₂ and irradiance in the range of 500–1200 µmol·photons·m⁻²·s⁻¹ had synergistic interactive effects on the POC and particulate organic phosphorus (POP) concentrations, while had antagonistic interactive effects on the Chl a concentration, PON , biological silicon (BSi) concentrations, elemental molar ratios, phytoplankton sinking rate and the abundances of diatoms and dinoflagellates. The results further reveal that the change of pCO₂ between 41 Pa and 101 Pa, and the irradiance in the range of 350–1200 µmol·photons·m⁻²·s⁻¹ can cause changes in the species composition of phytoplankton. High light (HL) favored the growth of the centric diatoms, while the abundance of Synechococcus was higher under OA. The abundance of dinoflagellates was decreased under OA and HL, while the abundance of Chrysophyceae and Cyanobacteria were both increased under OA. At the same time, it also hadan impact on the particle size composition of phytoplankton communities, OA significantly increased the percentage of micro-phytoplankton biomass and leads to the increase in both size and the sinking rate of phytoplankton community, thus having potential impacts on the carbon export and the formation of anoxic zones in the Yangtze River Esturay area.