Abstract: The hydrodynamics in a three-phase air-lift internal-loop reactor were studied in terms of gas holdup, liquid circulation velocity and residence time distribution. The results showed that gas holdup was increased with superficial gas velocity and solid holdup. The solid holdup played an important role in gas holdup at lower superficial gas velocity, whereas the solid holdup had a little effect on gas holdup at higher superficial gas velocity. On the basis of lots of experimental data, the experienced correlation for predicting gas holdup was obtained. The predictable values of gas holdup were well fitted with its experimental values. A mathematical model for predicting liquid circulation velocity was proposed in terms of the balance between driving and resistance forces. The curve of residence time distribution function was gained by using pulse chasing method, and it was known that the reactor had approached completely mixing reactor in terms of computing mean of residence time, dimension and dimensionless variance of residence time distribution and so on.