As a promising photocatalyst driven by visible light, BiOI suffers D from its lower conduction band edge position, which leads to its inability to produce hydrogen from photocatalytic water splitting. However, BiOI has an open layered intergrowth structure, which makes it easily cleavable along (001) plane. Thus, inspired by the progress of graphene-like two-dimensional nanomaterials, researchers believe that single-layer BiOI presents excellent photocatalytic activity for water splitting. To further explore the relationship between intrinsic properties and photocatalytic performance of BiOI ultrathin film, its electronic structure and optical properties as a function of layer thickness are systematically investigated by using first-principle calculations. The calculated results indicate that the quantum confinement effects can cause the following variations: band gap increasing, band edge position upshifting, and built-in electric field strengthening, which are very favorable for enhancement of photocatalytic performance. Importantly, if the layer thickness is less than 3 nm, the conduction band edge position will be higher than the reduction potential of H+/H-2 and thus appropriate for the overall photocatalytic water-splitting reaction. However, layer thickness also caused disadvantageous reduction of sunlight absorption, which is noticed and avoided in practice.