It is an efficient way to use the solar energy for photocatalytic H₂ and O₂ evolutions on semiconductors to resolve the problems of fossil energy shortage and environmental pollution. The photocatalytic systems of H₂ and O₂ evolutions are very complex and involve a series of issues in science and technology for obtaining the efficient photocatalytic systems of H₂ and O₂ evolutions, among which the selection of co-catalyst is a crucial one. This paper focuses on the progresses of the co-catalysts of photocatalytic H₂ and O₂ evolutions, analyzes and introduces the roles of cocatalysts integrating with the principle of photocatalysis. The roles of the cocatalyst as the active sites for H₂ and O₂ evolutions include the catalysis in the reactions of H₂ and O₂ evolutions, the reduction of overpotentials and the efficient separation of photo-induced electrons and holes. This paper also introduces the idea of the development of cocatalyts in the photocatalytic systems in the future. It is believed that the efficient combination of dual and multiplex cocatalysts, and the exploration of the non-noble metal cocatalyst are the direction of development in the future.