Abstract: Euler-Euler two equations model was applied to model the gas-liquid two-phase flow in an airlift reactor.The effect of superficial gas velocity on the liquid recirculation velocity and gas holdup were investigated.The achieved superficial liquid velocity and the gas hold up in the riser corresponded with the classical empirical correlations very well,which proved the validity of the CFD model in predicting the gas-liquid two phase flow in airlift reactors.Based on this,the effect of downcomer diameter and height in the reactor were investigated,liquid circulating velocity and gas hold-up in the riser increased with draft-tube diameter.Liquid velocity increased with draft-tube height,while gas holdup in the riser decreased with it.The results achieved are valuable for the design and optimization of this kind of reactors.