In this work, steady-state and time-resolved photoluminescence (PL) spectroscopy were applied to study the effects of CdS particle size on the charge transfer between CdS quantum dots (QDs) and cobaloxime (CoⅢ(dmgH)₂(3-(OH)py)Cl) in CdS QDs-cobaloxime hybrid photocatalytic hydrogen production system. All three kinds of CdS QDs with different particle size (3.9, 4.4 and 5.0 nm) show a band-edge emission and a broad trap-related emission. When the particle size of CdS QDs is smaller, the band-edge emission intensity is reduced while the trap-related emission enhanced, and CdS QDs with smaller particle size exhibit longer emission lifetime. Upon mixing with cobaloxime, the band-edge emission and trap-related emission of CdS QDs are quenched quickly, and the quenching constant of the trap-related emission is bigger than that of the band-edge emission. The emission intensity of CdS QDs is quenched more efficiently with the particle size decreasing. These results demonstrate that, both the free and trapped charge carriers of CdS QDs could transfer to cobaloxime, and the trapped ones transfer to cobaloxime more efficiently; CdS QDs with smaller particle size exhibit higher charge transfer efficiency.