Abstract: Deep native fissures are ubiquitous in expansive soil.Deep sliding along the fissure surface is an important mode of slope instability in expansive soil.In order to understand the deformation evolution characteristics of expansive soil high slope under deep fractured instability mode, a prototype test of high slope excavation unloading was carried out.The horizontal displacement of the slope developing from stable to failure was tested, and the geometry of slip surface was obtained, so that the mechanical mechanism of deep fractured instability mode was derived.The displacement rate exponent was used as the quantitative parameter to describe the slope deformation evolution trend, and the influence of excavation rate and rainfall on slope deformation state was analyzed.According to the trend of deformation development, slope deformation is divided into three state categories: slow instability, long-term instability and rapid failure, and their quantitative thresholds are proposed; according to the distribution law of slope displacement rate and its power exponent under different deformation states, a method for predicting deep fissured landslide in expansive soil is proposed.The results provide theoretical and experimental basis for deformation control and landslide early warning of expansive soil slopes.