Mineralogy and genesis significance of mixed-layer illite-vermiculite in Xuancheng red soil section were investigated using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and inductively coupled plasma emission spectrum (ICP-AES). XRD shows that clay minerals in the red soils are composed of illite, vermiculite, mixed-layer illite-vermiculite, and kaolinite. The lattice fringes of mixed-layer illite-vermiculite were directly observed under HRTEM. Vermiculite layers were trapped within the illite layers, indicating that mixed-layer illite-vermiculite was derived from illite. The higher content of vermiculite, mixed-layer illite-vermiculite, and kaolinite in the net-like red soil relative to the homogeneous red soil suggested a warmer and more humid climate prevailing over the mid-Pleistocene and a relatively cool and dry climate prevailing over the late Pleistocene. A lower hydroxy-Al hydroxides content of the vermiculitic layers in the net-like red soil compared to those of the homogeneous red soil resulted in more tortuous lattice fringes. Hence, Mixed-layer illite-vermiculite with hydroxy-Al hydroxides interlacated is sensitive to the climatic environment change in soil-forming process.