Abstract: It is necessary to study the effect of ocean acidification (OA) on Pyropia haitanensis which is an important economic species ecologically. In this study, we investigated the physiological and ecological effects of ocean acidification on P. haitanensis thalli by simulating ocean acidification with an aerated culture of CO₂ enriched-air (1 000×10^-6).The results showed that the specific growth rate was significantly decreased by 14.8% when incubation density of 1 cm thallus increased from one thallus per litre to three thalli per litre. At condition with low incubation densities, OA reduced the specific growth rate by 6.7%. On the contrary, at condition with higher incubation densities, OA increased the specific growth rate by 6.7%. This suggests that higher density incubation can partially alleviate the negative effects of OA. Regardless of the incubation density, OA did not affect significantly on Fv/Fm, total protein content, pigment content, and carbon, nitrogen and phosphorus contents of algal tissue while it significantly reduced the release of particulate organic carbon (POC). In particular, under lower density incubation, OA increased significantly the release of dissolved organic carbon (DOC) by 70.2%. In summary, the effects of OA on the carbon accumulation rate and organic matter release from the thalli were dependent on the incubation density.