The logarithmic spiral failure surface was used to analyze the slope and the upper soil. Based on the principle of energy conservation, assuming the rotation angle dθ, a stability calculation model of the slope was established by the radial slice method. Based on the rotation angle dθ, the gravity work, the friction energy dissipation on the sliding surface and the energy dissipation due to the deformation induced by inner friction between blocks were deduced. The safety factor F_S was calculated by using the strength reduction method. Through an example and comparing the calculation results with those by the limit analysis method, it is shown that the calculation results of the energy method and the limit analysis method are rather close, and the former has a lower stability coefficient. The stability calculation analysis on the main parameters in the model also shows that both methods are close and the energy method is more convergency. The results have certain referential value for the stability evaluation and design of the slope.