The locked section is a kind of geological structure that is commonly found in natural slopes. The instability criterion of the slope with a locked section structure is not yet studied. In accordance with the mechanical properties of this type of slope, the rock mass is divided into several rock blocks along its joint distribution, and the upper limit method of plastic limit analysis is introduced. By combining the principle of virtual work and the characteristics of the slope, a virtual speed is assigned to the slope and the locked section. In addition, the maximum elastic strain energy that can be accumulated in the locked section is calculated to derive its deformation rate and internal energy dissipation power. The virtual velocity, internal energy dissipation power, and gravity work power of the locked section and the rock slope are solved simultaneously in accordance with the Mohr–Coulomb correlation flow law. Finally, the formula for calculating the safety factor of this type of slope under shear failure mode is derived. A slope in Xikou, Sichuan, China, is taken as an example to verify the proposed model.