Graphene/CdS composites were synthesized through the direct deposition of CdS nanoparticles on graphene sheets. The high conductivity of graphene sheets and the intimate heterointerfacial connection between graphene sheets and CdS nanoparticles provided prominent advantages for enhancing light absorption and facilitating the transfer of photogenerated carriers from CdS nanoparticles, thus leading to an effectively separation of electron-hole pairs and consequently an improvement in photocurrent intensity. A highly sensitive and selective photoelectrochemical sensor for detecting copper ions (Cu2+) was developed based on the interaction between Cu2+ and CdS by forming CuxS-coated CdS nanoparticles, which serves as the recombination centers, impedes the transfer of electron from the conduction band of CdS to graphene sheets, and consequently leads to a decrease in photocurrent intensity. The sheets not only effectively transferred the photogenerated electrons deriving from CdS nanoparticles but also resulted in an enhancement in photocurrent intensity in the presence of various metal ions except Cu2+. The sheets amplified the photoelectric response of CdS semiconductor for Cu2+ sensing, in which the photocurrent intensity decreased dramatically.