Despite the excellent photocatalytic activity of MoS2/CdS heterostructures is known to be directly related to the orientation of contact surfaces, the origin is unclear. In the present work, we investigate by means of first principle calculation how the photocatalytic activities of MoS2/CdS(0 0 1) and MoS2/CdS(1 0 0) are the facet dependent. The results reveal that comparing to MoS2/CdS(0 0 1), MoS2/CdS(1 0 0) displays expanded light absorption edge, fast migration velocity and higher visible absorption ability. We trace this surprising result to the generation of type-II heterojunction which lead to the reduction of band gap to 0.335 eV, making it easier for photoelectons to move from CdS to MoS2 monolayer. And, MoS2/CdS(1 0 0) have suitable band edge and is prone to absorb H2O taking off the H-2 molecules, which is an apt water-splitting photocatalyst for hydrogen generation. The present work provides theoretical information to reveal the intrinsic nature of MoS2/CdS in terms of facet-dependent photocatalytic performance, and such an approach can be used to explore the surface effect of other heterostructures of transition-metal semiconductors.