Synergetic experimental and DFT insights of energy band structures and photogenerated rate-limiting reactive species are indispensable to design impurity-doped photocatalysts for photocatalytic environment remediation and solar fuels. Herein, despite the larger bandgap (Eg) of the Zn-doped BiOBr samples, they exhibited superior activity to BiOBr in the photocatalytic water splitting but impaired the photodegradation of Rhodamine B under visible-light illumination. Based on the spectral and electrochemical impedance characterisations and DFT simulations, the wider bandgaps of Zn-doped BiOBr samples were explicitly assigned to the more positive valence band maxima (VBM) and more negative conduction band minima (CBM). The enhanced photocatalytic water splitting on the Zn-doped BiOBr was arisen from the higher redox chemical potentials of charge carriers on respective CBM and VBM, suppressed back reactions and depressed recombination of photogenerated charge carriers. However, the reduced e(-)-h(+) recombination on the Zn-doped BiOBr cannot cancel the detrimental influences from the weaker light absorption and dye-sensitisation effects, leading to slower RhB photodegradation.