Abstract: The formation process of weak interlayers within red bed accumulation is a complex and long-term process, and analyzing the water-rock(soil) chemical action inside the red bed accumulation is an effective means to understand the formation process of weak interlayers.Taking a large red bedded landslide, Guanping ancient landslide, as an example, based on geological investigation and sampling, ESR geological dating, X-ray diffraction analysis, groundwater quality analysis and physical and mechanical tests, the characteristics of water-rock(soil) chemistry, the causes of weak interlayer formation and physical and mechanical properties of the red bedded landslide have been investigated.The results showed that the ancient landslide and its weak interlayer were basically the products of the same period, the main minerals of the weak interlayer were clay minerals, and the landslide and its weak interlayer were homologous in term of mineral composition; the weak interlayer was formed by the enrichment of the products of the water-rock(soil) chemical reaction of the red layer accumulator transporting to the water flowing cross-section, and the thickness of the weak interlayer was directly proportional to the time of the water-rock(soil) chemical reaction; physical and mechanical tests showed that the weak interlayer was a slippery stratum in the red layer of the accumulation body.