Copper-based ternary semiconductors have been applied as hole-transporting materials (HTM) for perovskite solar cells (PSCs) in recent years. However, the unavoidable defects of inorganic nanoparticles affect the performance of PSCs. Here, we prepared CuGaS2 quantum dots (CGS QDs) and employed them as HTM to PSCs. The Cu content of CGS QDs was precisely controlled and its influence on the photoelectric properties of the solar cells was systematically investigated. An optimal power conversion efficiency (PCE) of 17.56% was obtained as the Cu content is 0.8:1 (Cu:Ga, molar ratio). Photoluminescence and electrochemical impedance analyses indicate that the surface defects of Cu-deficient CGS QDs were significantly reduced, thus resulting in effective suppression of carrier recombination between solar cell interfaces. Moreover, the stability of CuGaS2-based PSCs is promoted obviously compared with that of the device utilizing Spiro-OMeTAD as HTM. This work demonstrated that reducing the Cu content of CGS QDs can effectively improve the PCE of CuGaS2-based solar cells, and CGS QDs with Cu-deficiency can be applied as a potential HTM to PSCs.