Under tectonic stress, the rocks in the fault zone are subjected to a loading-unloading cycle, where their micro-and meso-scale damage deterioration directly impacts the deformation stability of engineering rock masses. Aiming at the problem of microscopic evolution of fault rock under tectonic stress, the sandstone in the Longmenshan fault zone of Sichuan Basin was loaded to different stress amplitudes by a rigid testing machine at a constant loading rate, and then unloaded to obtain rocks with different damage degrees. The microstructural changes of sandstone after different damage were analyzed, which broke through the bottleneck of non-uniform sandstone parameters and microscopic damage characterization. The evolution law of micro-cracks in the process of sandstone deterioration was quantitatively analyzed from the mineral particle scale, and the process of sandstone damage and deterioration is divided into three stages. The first stage of deterioration: new cracks dominate, and the direction of cracks is randomly distributed. The second stage of deterioration: the main deterioration mechanism is the expansion of new cracks and original cracks, and the micro cracks expand along the direction of principal stress. The third stage of deterioration: the main deterioration mechanism is the accelerated expansion of microcracks, and the microcracks continue to expand along the direction of principal stress and penetrate each other. The research results can provide a reference index for earthquake disaster early warning.