Considerable effort has been made to fabricate graphene (GR)/semiconductor composite photocatalysts, by using graphene oxide (GO) as the most widely used precursor of GR, toward an improved efficacy of solar energy conversion. However, thus far, the role of GO in the preparation and photocatalytic activity of GR/semiconductor composites has remained rather elusive. Herein, we report a simple yet efficient approach to significantly improve the photocatalytic activity of GR/semiconductor CdS composites via the acid treatment of GO, which downsizes GO sheets into smaller ones with enhanced colloidal stability and oxygenated functional groups. The graphene/CdS composites, which are prepared using this type of downsized GO as the precursor of GR, exhibit remarkably higher visible-light photoredox activity than those prepared from the direct reduction of GO without acid treatment. Our comparative results directly highlight the important effect of physico-chemical features of GO on the preparation and thus photoactivity of GR/semiconductor composites; in particular, the rational tailoring of GO could open a new doorway to optimize the activity of GO-derived GR/semiconductor composite photocatalysts.