Abstract: The chemical activity of ions in salt solution will change the mechanical properties of saturated geomaterials, inducing geotechnical problems such as deterioration or even failure of engineering properties. In order to study the effects of chemical activity on the hydraulic and deformation properties of saturated geomaterials, a constitutive theory framework for geomaterials saturated with salt solution is established based on the mixture theory and thermodynamic solution theory. Different from the previous studies, the solid phase strain is decomposed into the skeleton one caused by porosity change, the matrix one caused by solid material deformation, and the mass exchange one caused by material exchange such as chemical reaction, to highlight the key role of porosity in the hydro-mechanical-chemo multi-field coupling mechanism. This theory adopts the current mass fraction of solute as the chemical state variable to reflect the effects of chemical activity. The free energy and dissipation potential are used to establish the elastic and plastic constitutive relations, respectively. Based on the above theoretical framework, the constitutive relations of solid and fluid phases for the clay saturated with NaCl solution and the seepage-diffusion equation for the solute are established. The constitutive model is validated by the experimental data, which proves that the framework can guide the establishment of the constitutive model for the geomaterials saturated with salt solution.