One of the current key issues in the photocatalysis is to obtain the band alignment of coupled semiconductor experimentally, which is the most significant aspect for understanding the efficient photocatalytic activity precisely. In this article, a BiVO4/BiOI{001} heterojunction was detected by the transmission electron microscopy (TEM). The results of electrochemical impedance spectroscopy (EIS) reveal that a higher efficiency of charge immigration across the electrode/electrolyte interface for the BiVO4/BiOI{001}. In addition, the electron lifetime was investigated by the Bode phase plots, which are responsible for the observed photocatalytic behaviors. As a result, the photocatalytic activities for rhodamine B (RhB) degradation and iodine ion oxidation are remarkably increased compared to the individual BiVO4 and BiOI{001}. In fact, a direct band alignment in the BiVO4/BiOI{001} does not give well a logical interpreting on its photocatalytic behavior. However, through the valence-band offset formula, we demonstrated the exact band alignment of corresponding components in the BiVO4/BiOI{001}. This band alignment was further confirmed by the PL spectra, which CM2 with photo-oxidation (Mn2+) is red shift and CM2 with photo-reduction (Ag+) is blue shift compared with CM2. •BiVO4/BiOI{001} heterojunction were prepared via a facile in situ chemical transformation approach.•The prolonged electrons lifetime of BiVO4/BiOI{001} is responsible for the observed photocatalytic behaviors.•We obtain experimentally the band alignment of BiVO4/BiOI{001} through the valence-band offset.•This band alignment was evidenced by the PL spectra (BiVO4/BiOI{001}.