Under atmospheric environment conditions, the natural oxidation of expansive soil may occur, and the chemical composition, mineral composition and particle structure of clay particles may change. In order to study the effect of atmospheric oxidation on the physical properties of Liulin weak expansive soil, the chemical components of undisturbed soil and oxidized soil clay were compared by XRF component detection and laboratory geotechnical test methods. The shear deformation and failure modes of undisturbed soil and oxidized soil samples were studied under different confining pressures. The hardening characteristics of soil samples under compression and shear failure modes were analyzed by the principal stress-strain constitutive relation. The results show that the key factor is the change of the internal friction angle for the increase of the shear strength index. Under the experimental loading conditions, the failure modes of undisturbed soil samples and oxidized soil samples are different, and various failure modes may occur, such as buckling compression, hardening compression, compression and shear mixing, or shear slip. For soil materials with softening properties, the hardening coefficient can be used to describe the nonlinear characteristics of the sample transition from compression hardening to compression shear mixing and shear softening failure mode. This paper can provide a theoretical reference for the study of the oxidation effect of expansive soil and the analysis of its engineering stability.