Abstract: The associated flow model based on the traditional elastic-plastic theory can not describe the constitutive relationship of soils well. It is because the direction of plastic strain increment of soils is not unique, which has been theoretically and experimentally proved. The traditional elastic-plastic theory is based on the assumption that the direction of the plastic strain increment is unique, so neither the associated flow rule nor the non-associated flow rule can solve the modeling problem of soil constitutive relationship satisfactorily, and it is necessary to develop a new theory. What is more, the known results show that the direction of the plastic strain increment of soil not only depends on the total stress but also the stress increment, which exhibits the characteristics of elastic strain. Based on the generalized potential theory, the decomposing rule of plastic strain increment is studied, and an elastic-plastic model for soils considering quasi-elastic-plastic deformation is proposed. The traditional unrecoverable plastic strain increment is decomposed into quasi-elastic part and pure-plastic part. The quasi-elastic part obeys the elastic rule, which has the same direction with the stress increment and can be expressed by elastic model. The pure-plastic part obeys the traditional plasticity theory, in which the direction is unique and the modeling of constitutive relationship can be based on the the assumption of plastic theory. The proposed model is more reasonable and convenient, and it can solve the problem of non-uniqueness of the direction of the plastic strain increment of soils. Finally, the feasibility of the proposed model is verified through comparison with the test results, indicating that the proposed model has better effectiveness.