In this work, a novel three-phase and three dimensional CFD model based on the Euler-Euler approach developed for modeling the hydrodynamic, mass transfer and CO2 fixation using microalgae in an internal loop airlift reactor with internal sparger. The main objective of this work was to development of CFD codes in order to simulate the CO2 biofixation process under different input gas velocity during 11 days of culture time and simulate the unsteady state mass transfer based on Surface-Renewal-Stretch (SRS) model. The CFD results were compared with the our previous experimental work and they showed good agreement with a margin of less than 10%. This paper illustrated the ability of the CFD in complex process simulation such as CO2 biofixation in the airlift reactor and provided a useful basis for further study.