A horizontal-tube segmentation-separation apparatus has multiple parallel inclined plates, which are arranged in pairs having an upper inclined plate and a lower inclined plate. Between the inclined plate pairs are isolated plates which are parallel thereto. One or more baffles are set under the isolated plates transverse to the flow of water to block the flow of water under the isolated plates. Between the isolated plate and the upper inclined plate is a flat supporting plate which separates the water flowing region into several water channels that have a parallelogram-shaped cross-section.