Ordered 2D nanostructural BiOI nanoflake arrays decorated with Bi2S3 nanospheres have been designed and in situ fabricated for the first time, to form BiOI/Bi2S3 bulk heterojunctions through a soft chemical route. A modified successive ionic layer adsorption and reaction (SILAR) method was developed to fabricate BiOI nanoflake arrays on flexible ITO/PET substrates at room temperature. The degree of transformation of BiOI to Bi2S3 was controlled through the adjustment of exposure time of the BiOI/ITO substrate to thioacetamide (TAA) aqueous solution. The morphologies of BiOI, BiOI/Bi2S3 heterojunctions and Bi2S3 films were examined by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) patterns, and high resolution transmission electron microscopy (HRTEM). The presence of Bi2S3 was further validated through Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Especially, photoelectrochemical measurements demonstrated that such a Bi2S3 decorated BiOI photoanode based cell exhibits significant augments of short-circuit current density (Jsc) and incident photon-to-current conversion efficiency (IPCE, 3 times higher than the pure BiOI photoanode), attributable to the stronger photo-absorption and better photogenerated charge carrier separation and transport efficiency. The surface photovoltage (SPV) measurements further confirmed the importance of BiOI/Bi2S3 heterojunctions in such PEC cells. This solution-based process directly on flexible ITO offers the promise for low-cost, large-area, roll-to-roll application of the manufacturing of the third generation thin-film photovoltaic devices.