Abstract: In order to investigate the effect of intermediate principal stress on the progressive damage and permeability characteristics of unloading sandstone, the unloading test was carried out under different intermediate principal stress conditions based on a multifunctional true triaxial fluid-solid coupling test system with gas-bearing sandstone as the research object; the exponential permeability model related to the cracked volume strain and damage variable were obtained. The results show that the crack closure stress, crack initiation stress and damage stress increase with the increase of intermediate principal stress, and the percentage of elastic section and microcrack extension stage in the pre-peak strain increases. The deformation in the direction of the intermediate principal stress is restrained in the microcrack extension stage, and the compression effect on the direction of the minimum principal stress is enhanced with the increase of the intermediate principal stress; in the macroscopic crack extension stage, the damage stress is closer to the peak stress, and the damage of the specimen is brittle in character. With the increase of the intermediate principal stress, the initial permeability reduction shows a decreasing trend and the point of elevated permeability shifts backward.