Abstract: There are some limitations in classic shear fracture criteria such as Coulomb-Mohr criterion and sliding friction law represented by Byerlee law. On the base of Coulomb-Mohr criterion and Byerlee law, starting from the physical nature of shear fracture, and applying the activation criterion of pre-existing weakness and the theory of activation tendency analysis, a generalized shear activation criterion is proposed. The criterion can be used to quantitatively determine the probability of shear activation for any medium, any stress state, any orientation plane. It extends not only Coulomb-Mohr criterion from homogeneous media to inhomogeneous media with pre-existing weakness, but the sliding friction law represented by Byerlee law from 2D stress state to any triaxial stress state, and shows broad application prospects.