Abstract: The interlocked action of stone asperity on transfixion sliding surface plays an important role in rock slope stability, but its mechanism and strength is not studied clearly. Here, based on rock mechanics and reasonable assumption, the stone unlocking models are analyzed first. Then taken classical friction law and upper bound theorem as measures, both the mechanisms and the condiations of two unlock models of Leap-slide and Shear-break are studied from the view of friction and energy respectively. Moreover, the shear strengths of interlocked stone under different loads are also given. At last, an example is given and the result shows that: 1) the stones’ stability and unlock model are determined by the relative value of sliding force Twith normal load Pand their absolute value. When T/Pis bigger than the critical value ξ, the stones will unlock by Leap-slide. On the contrary, when T/Pis small but Tis big enough, the stone will be sheared break. Otherwise, the stone will keep interlocking; 2) Leap-slide is the result of sliding friction of occlusal surface, and Shear-break is the process of energy dissipation of external work on potential failure surface; 3) the interlock strength is the reflection of critical mechanism coditions when stone asperities unlock under different external loads.