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The carbonic anhydrase (CA) is considered as an important enzyme in the process of primary production of phytoplankton.We present the results of a field study in the northern South China Sea (NSCS) during the northeast monsoon season (2010 Nov.).The enzyme activity of CA was investigated through the effect of the specific CA inhibitors acetazolamide (AZ) and ethoxyzolamide (EZ) on the P-I curve parameters.The results showed two patterns:(1) in the regions of coastal water,the inhibitor EZ mainly affects the initial slope (α) of P-I curve to reduce the photosynthetic rate (～50％ inhibition when α ＞ 0.03);(2) in the regions of deep water,the inhibitor EZ caused more severe photo-inhibition in the high irradiance levels the phytoplankton was exposed to in the sea surface.And the effect of inhibitor AZ showed more variability within different regions and size-fractions.The results revealed that the different roles of CA depending on the diversity of phytoplankton assemblage structure.Through the comparison with the data we got in other seasons,we considered that the reduction of photo-inhibition by the together effect of higher intracellular CA activity and deeper mixing layer depths was the important factors for the higher primary production in the northeast monsoon season than in the southwest monsoon season in the NSCS basin.