[en] Highlights: • Suitable band engineering is important in separating photoinduced electron-hole pairs. • Based on the band engineering theory and the quantum size effect, we designed a band-gap-matched photocatalyst. • This photocatalyst shows high-efficiency photocatalytic activity (14 mmol h^-1) and high-stability. Suitable band engineering is required in order to develop new photocatalysts for water splitting under visible light irradiation. Based on the quantum size effect, the band gap energy varies with the size of the quantum dots (QDs) tuning. Besides, tuning the band gap energy of CdSe QDs can match well with the band gap energy of WS₂ nanosheets, which is benefiting the separation of electron-hole pairs generated in CdSe QDs under irradiation. Some links exist between the photocatalytic activity and band gap energy of QDs. The highest rate of hydrogen evolution under visible light irradiation is 14 mmol h⁻¹ with 58% quantum efficiency at wavelength λ=420 nm when the QDs size is 7–8 nm and the band gap of QDs is 1.48 eV. Designing and fabricating band gap energy matched nanocomposite photocatalysts can present potential applications in solving future clean energy problems.