The morphology of catalyst is a very important factor influencing the photocatalytic activity of the catalyst. Owing to its scientific and technological importance, controllable preparation of photocatalysts with different morphologies has been studied. In this work, BiOIO3/g-C3N4 heterojunction composites are synthesized using hydrothermal method and the composites with different morphologies are fabricated by adjusting the amount of NaOH in precursor solutions to control the growth of BiOIO3 crystal plane. The physicochemical properties of BiOIO3/g-C3N4 heterojunction composites were investigated by XRD, XPS, FTIR, SEM, TEM, HRTEM, BET, UV-vis DRS and PL characterization. The effect of the BiOIO3 morphology on the photocatalytic efficiency of BiOIO3/g-C3N4 heterojunction composites was evaluated by photocatalytic removal of gas -phase Hg under visible light irradiation. When the morphology of BiOIO3 is regular square-like, BiOIO3/g-C3N4 has the optimal removal efficiency 92.6% of He. Finally, electron -hole migration path and photocatalytic mechanism of catalysts are proposed. (C) 2019 Published by Elsevier Inc.