The electronic structures of two dimensional (2D) CdS/ZnO heterostructure (CdZnHT) consisting of CdS singlelayer (SL) and ZnO SL are explored based on hybrid density functional calculation. The negative interface formation energies suggest the formation of CdZnHT is exothermic. The bandgap of CdZnHT is favorable for absorbing visible light, and the decent band edge position makes it thermodynamically feasible for spontaneous generation of oxygen and hydrogen. The formed electric field across the interface induced by charge transfer will reduce photogenerated carrier recombination and promote carrier migration. Particularly, CdZnHT is a type-II heterostructure. Oxygen generation takes place at ZnO layer and hydrogen production occurs at CdS layer, which will also promote the effective separation and migration of phogogenerated carriers and enhance photocatalytic performance. These findings suggest that 2D CdZnHTs are possible candidates as water-splitting photocatalysts.