Diatoms are one of the most important groups of phytoplankton in terms of abundance and ecological functionality in the ocean. They usually dominate the phytoplankton communities in coastal waters and have many strategies to accommodate the frequent and large fluctuations in light such as change their cell size. In this study we culture Thalassiosira weissflogii under pCO2 (400ppmv, 1000ppmv) under different light levels (100, 200, 400 μmol m2-s-) to examine its physiological performance. It showed that under high light condition (HL, 400 μmol m2-s-), the cell size was smaller about 3% than low light (100μmol m2-s-), however, the relative growth rate was higher about 33.6% and 22.7% than low light (LL, 100 μmol m2-s-) and medium light (ML, 200μmol m2-s-) respectively. There had no significant difference between LC and HC under different light conditions. The ETR(II)400 , rETR and net photosynthesis rate increased with the increasing light intensities, and the their maximum value were found in HL condition. And the contents of Chla in high light were lower than other two light levels, while the Car/Chla was higher than them. We verified that down-regulation of CCM occurred under low light level and HC condition by measuring the activity of external carbonic anhydrase (eCA). The linkage between higher level of the CCMs down-regulation and higher growth rate at HL under OA supports the theory that the saved energy from CCMs down-regulation adds on to enhance the growth of the diatom.