A semiconductor nanowire-based photosensor includes a substrate, at least a top surface of the substrate being formed of an insulator, two electrodes spaced at a predetermined interval apart from each other on the substrate, metal catalyst layers disposed respectively on the two electrodes, and visible-range semiconductor nanowires grown from the metal catalyst layers on the two electrodes. The semiconductor nanowires grown from one of the metal catalyst layers are in contact with the semiconductor nanowires grown from the other metal catalyst layer, while the semiconductor nanowires grown respectively from the metal catalyst layers on the two electrodes are floated between the two electrodes over the substrate.