The production of renewable fossil fuels such as CH₄ and CO by photocatalytic CO₂ reduction has attracted more and more attention. However, single photocatalyst is less efficient for photocatalytic reduction of CO₂ due to the fast recombination of photogenerated electron pairs. Herein, we successfully prepare CdS-Ag₂S composite by assembling the Ag₂S QDs cocatalyst on the surface of CdS nanosheet-assembled flower through oil-bath solvothermal method. This composite is prepared through a simple self-assembly strategy using cadmium chloride, ammonia and thiourea as precursors of the CdS nanosheet-assembled flower and silver nitrate and 3-mercaptopropionic acid as the precursors of Ag₂S QDs. The average diameter of Ag₂S QDs is apparently 6.0 nm. The light absorption edge of the composite is at around 560 nm, with the corresponding band gap at 2.14 eV. The CdS-Ag₂S QDs composite with 5 wt% Ag₂S QDs loaded achieves CO evolution rate of 16.6 μmol·g^-1·h^-1 without noble-metal cocatalysts. This strengthened photocatalytic performance and photocatalytic stability were attributed to the energy band broadening of Ag₂S QDs caused by quantum size effect and the large specific surface area due to the assembled flower. The mechanism underlying the enhanced photocatalytic CO₂ reduction activity is further proposed. This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis.