Abstract: On the basis of the framework of internal variable quasi-thermodynamics,the relationship between viscosity theory and plasticity theory is studied.Assuming that the reference point at the reference equivalent time line is an initial loading point and the effective stress is abruptly loaded on clay,some behaviors of the iso-time lines derived from Yin-Graham’s 1-D elastic visco-plastic model are analyzed.The scope that new Hypothesis A is approximately equivalent to Hypothesis B is obtained and the flaws of the original Hypothesis A are pointed out.When the loading effective stress is 1.1 times larger than the initial effective stress and the duration time is larger than the reference absolute equivalent time,the absolute equivalent time is approximately equal to the duration time.The strain of clay is the sum of plasto-elastic strain and fractional creep strain,in which the plasto-elastic strain depends on effective stress and is independent of time,while the fractional creep strain depends on time,but not on effective stress.By means of the theory of internal variable quasi-thermodynamics,the internal relation of Lagrange’s dissipative rate function between the 1-D EVP model and a pure plastic model is demonstrated and the reason why the 1-D EVP model has quasi-plastic visco-elastic characteristics is revealed.A 3-D EVP model is generalized from the 1-D EVP model according to the rule of Lagrange’s dissipative rate function occurring in the 1-D EVP model.Compared with experimental data obtained from consolidation-undrained triaxial shear creep test,the present 3-D EVP model is verified to be valid.