Abstract: In this paper,a three-dimension carbon cycle model,including several variables such as organic/inorganic carbon in the water,CO₂ in the water,carbon in phytoplanktons,fish,and phytobenthons,and organic/inorganic carbon in sediments and the water of clearances,is constructed based on the three-dimension dynamic model.In Taihu lake,the carbon cycle simulation results show that the average difference between the simulation values and the observation values is less than 40% in 2003,so it can be conclusded that the model can simulate the carbon cycle in Taihu lake and can be used to study the carbon cycle in the lake.At the same time,the results of the model show the carbon cycle in the lake can reduce the concentration of CO₂ in the air.