The effects of S-doped (2%, 4%, and 6%) g-C3N4 (S-CN) on the visible-light photocatalytic performance of CuxSey nanostructures have been investigated. In addition, the pristine CuxSey nanostructures, pristine g-C3N4 (g-CN), S-CN nanosheets, and CuxSey/g-CN nanocomposites have been also synthesized. A comparison of the results showed that CuxSey/S-CN nanocomposites performed the highest photocatalytic performance for the degradation of methylene blue (MB) dye under a visible light source and caused more than 80% of the dye molecules to degrade in just 30 minutes. In fact, it was observed, the CuxSey/S-CN nanocomposites in the first 15 min of the photocatalytic process, 5.5% more than the CuxSey/g-CN nanocomposites caused the degradation of dye molecules. Several important factors such as higher carrier, greater porosity, and formation of the Z-scheme have been recognized that have caused to enhance the photocatalytic performance of the CuxSey/S-CN nanocomposites in comparison to the CuxSey/g-CN nanocomposites. The Mott-Schottky and PL results confirmed that the CuxSey/g-CN and CuxSey/S-CN nanocomposites formed a type-II and Z-scheme heterostructure, respectively.