A large number of engineering practices show that the strength characteristics of rock under unloading conditions are essentially different from those under loading conditions. Combined with the failure characteristics and strength characteristics of rock unloading, the calculation model of rock bridge penetration force is established. Based on the principle of superposition and reduction of stress intensity factor, the calculation formula of rock bridge penetration force under rock unloading condition is derived to explore the applicability of Mogi-Coulomb strength criterion to rock unloading strength. Furthermore, the rock bridge penetration force is used as the strength parameter to characterize the unloading failure of rock, and compared with the core parameter of Mogi-Coulomb strength criterion-octahedral shear stress. It is found that the magnitude of the two is very similar. This finding can indirectly prove that the Mogi-Coulomb strength criterion is suitable for related research on rock unloading strength. Relevant research reveals the close relationship between rock bridge penetration and octahedral shear stress during rock unloading failure, which can provide reference for the study of rock unloading failure.