The checking of rock slope’s plane sliding includes the internal structure of rock mass surface checking and rock shear strength calculation. The sliding of homogeneous rock slope is due to insufficient shear strength. By assuming the relationship between the failure surface and the internal friction angle, we probe into the relationship between the failure angle and internal friction angle, and then propose the simplified estimation formula. Comparing the analytical solution with the numerical solution, we know that the simplified formula meets the engineering requirements, and the conclusion may serve as a reference for the practical engineering.