CdS/ZnS heterostructures with tunable band gaps are promising photocatalysts for solar- or visible-light-driven H2 production through water splitting. To predict how the bandgap changes with the heterostructure composition, density functional theory calculations with meta-GGA correction are performed. It is found that the band gaps of CdS and ZnS are reduced by up to 14.5% and 43.3% in the heterostructures, respectively. The content of CdS in heterostructures plays a vital role in tuning the band gap and conduction band edge level. With the increasing number of CdS layers, the band gap first decreases and reaches a minimum value for (CdS)5/(ZnS)5, and then increases slightly. As a result, the (CdS)m/(ZnS)n (m ≥ 3, m + n = 10, or ≥ 30% of CdS) heterostructures attain desirable band gaps in the range of 2.06-2.25 eV for visible light absorption and 0.305-0.444 eV more negative conduction band edge than the reduction potential of H+/H2 for water splitting. These results suggest that the composition of CdS/ZnS heterostructures can be adjusted to further improve the efficiency of photocatalysts for visible light absorption and water splitting/H2 production.