A better understanding of the stress–strain behaviors of shale samples after shale-CO2 or shale-water–CO2 interactions is of great importance to CO2 enhanced shale gas exploitation and CO2 sequestrating in shale reservoirs. In this study, a constitutive model that combines with the modified Duncan–Chang model and Weibull distribution-based model is applied to investigate the stress–strain characteristics of low-clay shale samples treated by sub-/super-critical CO2 and sub-/super-critical CO2 + water for different times (10 days, 20 days, and 30 days). The results show that the model could describe well the crack closure stage, the elastic stage, and the inelastic stage of shale samples. The axial strain at the connection point between the two models varies from 28.51 to 43.36% of the axial strain at the failure point. Shale-CO2 or shale-water–CO2 interactions make shale samples more ductile at the crack closure stage, which can be depicted as the increase of initial elastic modulus during the imbibition process. The brittleness index values (BI) which are calculated based on the combined constitutive model increase with increasing soaking time for shale samples treated by sub-/super-critical CO2, and decrease with increasing soaking time for shale samples treated by sub-/super-critical CO2 + water.