In this study,the Lakes Hongfeng,Baihua and Aha were studied as the research areas. We focused on the carbon isotopic composition of microalgae in response to global climate change by comparing the concentrations of the dissolved inorganic carbon(DIC),the activity of the extracellular carbonic anhydrase and the stable carbon isotope compositions in microalgae. From this study, we have got the following conclusions. Both the DIC and the activity of extracellular carbonic anhydrase were the key factors affecting the stable carbon isotopic composition of microalgae. It will be very important for the study of global climate change.