Abstract: There is increasing evidence that different light intensities or ocean acidification (OA) induced by elevated atmospheric CO₂ concentration can affect the photosynthetic capacity of marine diatom to different degrees, respectively however, little attention had been paid to their interaction on diatom. In this study, the growth rate, net photosynthetic rate (P_n), biochemical composition, extracellular carbonic anhydrase (eCA) activity, and Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubiscO) activity were investigated when Phaeodactylum tricornutum was grown under different light intensities and CO₂ concentrations. The results showed that the specific growth rates and P_n in P. tricornutum were not significantly affected by CO₂ concentration under low light intensity (LL), whereas in presence of the high light intensity (HL), elevated CO₂ concentration was beneficial to promote the increase of the rate of P_n. The eCA activity, chlorophyll content, and soluble protein content decreased with increase of CO₂ concentration, regardless of the high or low light. Under LL, RubiscO activity of HC-grown algae was 2.42 and 1.39 times higher than that of LC- or Medium-CO₂ (MC)-grown ones. However, RubiscO activity of HC-grown algae was 6.72 and 3.45 times greater than that of LC- or MC-grown ones under high light. These results indicate that the algae can adapt to changes of light intensity and CO₂ concentrations in the environment by adjusting the allocation of energy during the operation of the CO₂-concentrating mechanism and photosynthesis.