In this study, a thermal–hydraulic–mechanical–chemical (THMC) multi-field coupling triaxial cell was used to study systematically the evolution of gas permeability and the deformation characteristics of sandstone. The effects of confining, axial and gas pressure on gas permeability characteristics were fully considered in the test. The gas permeability of sandstone decreases with increasing confining pressure. When the confining pressure is low, the variation of gas permeability is greater than that of gas permeability at high confining pressure. The gas injection pressure significantly affects the gas permeability evolution of sandstone. As the gas injection pressure increases, the gas permeability of sandstone tends to decrease. At the same confining pressure, the gas permeability of the sample during the unloading path is less than the gas permeability of the sample in the loading path. When axial pressure is applied, it has a significant influence on the permeability evolution of sandstone. When the axial pressure is less than 30 MPa, it significantly influences the permeability evolution of sandstone. At axial pressures greater than 30 MPa, the permeability decreases as the axial pressure increases. Finally, the micro-pore/fracture structure of the sample after the gas permeability test was observed using 3D X-ray CT imaging.