Effective separation and migration of photogenerated electron-hole pairs are two key factors to determine the performance of photocatalysts. It has been widely accepted that photocatalysts with heterojunctions usually exhibit excellent charge separation. However, the migration process of separated charges in the heterojunction structures has not been fully investigated. Herein, photocatalysts with heterojunctions are constructed by loading g-C3N4 nanoparticles onto BiOCl nanosheets with different exposed facets (BOC-001 and BOC-010). The g-C3N4 nanoparticles with decreasing size and increasing zeta potential could induce stronger coupling and scattering in the heterojunction. The relationship between the crystal facet orientation in the BiOCl nanosheets and charge separation/effective migration behaviours of the materials is investigated. The visible light photocatalytic activity of the composites is evaluated by methyl orange (MO) and phenol degradation experiments, and the results show that ngCN/BOC-010 composites exhibit higher photocatalytic performance than that of ng-CN/BOC-001 composites. Both photoelectrochemical and fluorescence emission measurements indicate that the different exposed facets in ng-CN/BiOCl composites could induce the migration of the photogenerated electrons in different ways, but do not significantly alter the separation efficiencies. The separated electrons in ngCN/BOC-010 undergo a shorter transport distance than that of ng-CN/BOC-001 to reach the surface reactive sites. The study may suggest that the crystal facet orientation in polar semiconductors is a critical factor for designing highly efficient heterojunction photocatalysts.