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The ever-increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO2 have led to a severe energy crisis and climate change. Photocatalytic reduction of CO2 into fuels using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO2 can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO2 on the surface of a semiconductor-based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO2 reduction over semiconductor-based photoelectrodes are described, and the basic principles of PEC CO2 reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO2 are also presented.