Abstract: The red-bed soft rock has weak water permeability and strong hydrophilicity, and is prone to softening and disintegration under the influence of water content and hydrothermal(temperature and rainfall) changes, which brings challenges to engineering construction and geological environment in red-bed soft rock areas. This paper studies the disintegration characteristics and microscopic mechanism of red-bed soft rock under different water contents and various hydrothermal test conditions. It uses samples from the red-bed soft rock excavations of the slopes of the Jintang and Zhongjiang sections in Western Sichuan. It uses a self-made rainfall device and temperature control equipement to carry out the cyclic disintegration tests on samples with different initial water contents under multi-hydrothermal test conditions. Red-bed soft rock disintegration is studied from five aspects including macro-morphological evolution, micro-structure and mineral difference, disintegration coefficient, R value(disintegration mass loss rate), and particle analysis after disintegration. The test results show a certain interaction between the macroscopic morphology evolution characteristics of the red-bed soft rock and the changes in micro-structure and mineral differences. The clay mineral content can be the main internal cause of disintegration of red-bed soft rock and is positively correlated with disintegration. The degree of disintegration is closely related to initial water content and hydrothermal effect. The R values of the two red-bed soft rock samples range from 0.22% to 1.63% and 0.05% to 2.17%. The particle size distribution is a good indicator for the degree of disintegration of a red-bed soft rock. Higher disintegration is characterized by fewer large and more small particles, and the small particles are mostly concentrated between 0.075 and 0.5 mm. A combined analysis of the disintegration coefficient, R value, and final particle size distribution curve can be effective for quantitatively assessing the disintegration and explaining the disintegration evolution characteristics of red-bed soft rock in conjunction with micro-structure and mineral differences. The research results can provide a certain reference for the engineering construction and geological evaluation of Jintang, Zhongjiang, and other red-bed soft rock areas in Sichuan.