Abstract: It is firstly discussed that the theoretical basis of soil mechanics is not very perfect,and is still in a half-theory and half-experience stage,lacking in a unified theoretical basis.It is pointed out that the continuum porous medium soil mechanics,based on the multiphase porous medium theory,is a science used to describe the behaviors of soils in nature,which can be regarded as a unified theoretical basis of soil mechanics.The important role of thermodynamics and internal state variable in establishing constitutive equations based on the multiphase porous medium theory is then introduced.How to establish balance equations of the multiphase porous medium and how to select and determine the governing equations in soil mechanics are discussed.With regard to the complicated problem of unsaturated soils,a detailed demonstration and deduction of elasto-plastic constitutive equations for unsaturated soils considering the energy dissipating and plastic volume deformation of air is proposed.It adopts the multiphase porous medium theory,thermodynamics and internal state variable theory based on three generalized stresses,i.e.the effective stress of unsaturated soils(equation(22)),matric suction and air pressure as three independent stress state variables.