Abstract: The slow variability of the physical-mechanical properties of clay in atmospheric environment with normal temperature, pressure and concentration is studied. By taking the Zhanjiang clay as the research object, the soil color of atmospheric environment changes from blue-gray, green-gray into yellowish brown, brown under impact, and the degree of particle agglomeration is enhanced. Further, the plasticity and expansibility, shrinkage, sensitivity, structural yield pressure are all reduced. Owing to the atmospheric oxidation, the mechanical properties of the clay is strengthened, but its long-term mechanical stability is depressed and the damage is potential since the structural strength is weakened. A generalized structural model for the variability of clay by atmospheric oxidation is established and discussed. The results show that the variation of atmospheric environment leads to change of redox environment, the chemical reaction among water-soil and atmosphere is carried out and moved, and the connection ways of micro-fabric unit are changed. Furthermore, the microstructure is transformed and the stability of mechanical system is affected. Oxidation reaction prompts the valence of the iron ions in the soil to increase, and they migrate along with the aging reaction of colloidal iron oxide to the crystalline state under the concentration gradient and the adsorption energy of soil particle surface. Iron oxide exists in the form of the mixed iron replaced by the coated iron one, and the microstructure becomes flocculation structure with low plasticity, physical contact and weak structural strength. This study may provide a theoretical support for the prevention and control of geological and engineering disasters owing to environmental change.