A novel multistage internal-loop airlift reactor was realized by using a novel interstage internal. In this reactor, a gas layer is formed below the internal and has important effects oil the hydrodynamic behavior, The effects of the opening ratio of the internal for the gas and liquid channels and superficial gas and liquid velocities were experimentally studied in both cocurrent and countercurrent operations. The results show that the gas layer height increases with an increase in the superficial gas velocity for both cocurrent and countercurrent flows. With ail increase in the superficial liquid velocity, the gas layer height decreases for cocurrent flow, but increases for countercurrent flow. The opening ratio of the internal for gas channels has a much more significant influence on the gas layer height than that for the liquid channel. A mathematical model for predictions of the gas layer height was proposed based oil the balance between the pressure drops of the gas and liquid through the internal. A good agreement was obtained between the calculated and experimental data. This model can be used for the optimum design of the novel multistage internal-loop airlift reactor.