In the traditional plastic potential theory

there is an implied assumption that the directions of principal stress and principal plastic strain increment are coaxial. However

there are non-coaxial phenomena during the rotation of principal stress axis and the traditional plastic potential theory can not describe this phenomenon reasonably. A quasi-elastic-plastic constitutive model was proposed based on the generalized potential theory

in which the traditional plastic strain increment was decomposed into quasi-elastic part and pure-plastic part. The quasi-elastic part obeys elastic rule and the pure-plastic part obeys the traditional plasticity theory. The proposed model is reasonable and convenient; and it can also reflect the non-coaxiality of soil. The validation results of simple shear test show that the simulation effect of the quasi-elastic-plastic model is good. The directions of the principal stress and the principal plastic strain increment simulated by traditional elasto-plastic model (coaxial model) are coaxial; while the simulation results of quasi-elastic-plastic model (non-coaxial model) can reflect the non-coaxiality during the rotation of principal stress axis. It proves that the quasi-elastic-plastic model based on the generalized potential theory conforms to reality better than the traditional elasto-plastic model (coaxial model) and it gives an effective method to study the non-coaxiality of soil.