The cobalt doped cadmium selenide/biomass carbon spheres (Co-CdSe/BCS) photocatalyst is easily prepared using a transitory hydrothermal reaction lasting 40 min. The Co-CdSe/BCS photocatalyst is constructed using the in situ growth of Co-CdSe nanodots on the surfaces of the BCS. Doped Co2+ generates an electron trap, which can capture the photoinduced electrons, resulting in the efficient separation of the photoinduced electron-hole pairs. Furthermore, the introduction of carbon spheres enhances the optical absorption property and the ability for electron transport. Results of electrochemical characterization and photoluminescence spectra demonstrate that the Co-CdSe/BCS is able to efficiently separate electron-hole pairs and inhibit their recombination. The as-synthesized samples display high efficiency for the photodegradation of methylene blue dye under visible light. This study offers a new strategy to promote photocatalytic performance of the CdSe photocatalyst.