Visible-light responsive g-C3N4 catalysts were successfully prepared by the calcination of urea, i.e., the thermal polycondensation method, and a series of ZnS/g-C3N4 composite photocatalysts were then synthesized using a rapid microwave hydrothermal method. The physicochemical properties of the resulting g-C3N4 and ZnS/g-C3N4 catalysts were characterized via several microscopy and spectroscopy techniques. The reduction of Cr(VI) was used to evaluate the photocatalytic performance of the as-synthesized catalysts under visible light irradiation. The results showed that the calcination temperature substantially affected the crystallinity, specific surface area and photocatalytic performance of g-C3N4. The optimal temperature for thermal polycondensation in the g-C3N4 synthesis was 600 degrees C. The bulk g-C3N4 catalyst exhibited no Cr(VI) reduction ability, while a certain amount of ZnS nanoparticles coupling with g-C3N4 significantly improved the Cr(VI) reduction efficiency, creating a synergistic effect between g-C3N4 and ZnS. The simultaneous Cr(VI) reduction and rhodamine B decolorization by the ZnS/g-C3N4 composite catalyst were also accomplished. In addition, a Cr(VI) reduction mechanism under visible light irradiation was proposed.