Abstract: Geotechnical plasticity plays an important role in the design of geotechnical engineering because most of them are in an elasto plastic state. In this paper, the development of geotechnical plasticity is reviewed and some problems of applying the classic plastic mechanics to geomaterials are analyzed, and then its three hypotheses unfitting to the deformation mechanism of geomaterials are pointed out. By giving up these three hypotheses, a generalized plastic potential theory can be obtained from solid mechanics directly, and then the traditional plastic mechanics can be changed to a more generalized plastic mechanics, namely generalized plastic mechanics (GPM). The GPM adopts the component theory as theoretical base, so it can reflect the influence of transition of stress path, and eliminate the mistake that the direction of plastic strain increments is independent of the stress increment; it requires that the yield surface must correspond to but not coincide with the plastic potential surfaces,then the unreasonable phenomena such as excessive dilatancy caused by the normality of flow rule,and the error caused by the arbitrarily assumed plastic potential surfaces can be avoided. The yield surface theory, hardening rules and stress-strain relations of GPM are given. Based on the decomposition of stress increments, a GPM including the rotation of principal stress axes can be established, and the plastic deformation caused by the rotation of principal stress axes can also be calculated. It is pointed out that the yield condition is a state parameter, which is relevant to stress state and stress history as well as the properties of material. On the other hand, the yield condition is also a test parameter, and it can only be given by test. After the practical application, it is shown that the GPM can not only be applied to the modeling theory of geomaterials but also to other fields of geomechanics such as limit analysis.