A favorable interface for hybrid photocatalysts makes an important contribution in enhancing photocatalytic reactions. Herein we located sulfur and nitrogen co-doped carbon dots (SN-CDs) onto g-C3N4 nanosheets and achieved a novel SN-CD/g-C3N4 composite with a high visible-light photocatalytic reduction of 4-nitrophenol (4-NP). Compared to g-C3N4 nanosheets, the SN-CD/g-C3N4 composite possessed a wider light absorption. More importantly, the interaction of surface groups led to a favorable interfacial bonding between SN-CDs and g-C3N4 nanosheets, which facilitated the transfer of photoexcited electrons and holes, giving rise to an improved electron transport ability of the composite. In addition, the upward shift of the energy band of g-C3N4 nanosheets due to the location of SN-CDs led to a stronger reducibility of charge carriers. As a consequence, the photocatalytic performance of SN-CD/g-C3N4 has been remarkably improved in the photocatalytic reduction of 4-nitrophenol.