Novel g-C3N4/Bi5O7I heterojunction microspheres having excellent photocatalytic activity were successfully prepared by a one-pot ethylene glycol (EG)-assisted hydrothermal method followed by calcination in air. Bi5O7I microspheres grafted with g-C3N4 nano-sheets were formed and studied by electron microscopy. The morphological effects on g-C3N4/Bi5O7I photocatalytic capability with varied g-C3N4 contents were investigated and discussed. The g-C3N4/Bi5O7I microspheres exhibited much improved photocatalytic degradation performance under visible light irradiation compared with bare Bi5O7I and g-C3N4. The preparation conditions of g-C3N4/Bi5O7I microspheres with optimal photocatalytic ability for degrading methyl orange (MO) and rhodamine (RhB) were established. Analyses by both photoluminescence (PL) and photocurrent independently confirmed that photo-induced electron-hole pairs in the g-C3N4/Bi5O7I composite have been effectively created which was responsible for the observed photocatalysis. Based on the analysis of the experiment results, a Z-Scheme heterojunction photocatalytic mechanism was proposed. The excellent photocatalytic performance could be attributed to the effective charge separation at the interface between g-C3N4 and Bi5O7I and the enhanced visible light absorption. In addition, the photocatalytic mechanism was discussed on the basis of the relative band positions of these two semiconductor materials.