A modified square airlift loop reactor (MSALR) was developed. Experiments were performed to investigate the effects of the ratio of height and diameter of draft tube (H-r/D-r), and the ratio of cross-sectional area of riser and downcomer (A(r)/A(d)) on gas holdup, liquid circulation velocity and mixing time in the MSALR. A comparison of these characters as well as the total volumetric mass transfer coefficient was carried out between MSALR and round airlift loop reactor (RALR). Finally, the mixing time within the MSALR, RALR and square bubble column (SBC) was also studied. The results indicate that the hydrodynamic characteristics of MSALR are better when H-r/D-r = 11.2 or A(r)/A(d) = 0.695, and its characteristics of gas holdup and mass transfer surpass those of RALR. The average volumetric mass transfer coefficient K(L)a of MSALR is about 40% larger than that of RALR, except for extreme conditions.