An improved horizontal slice method (IHSM) for assessing the multi-layer slope stability that considers the energy dissipation between adjacent horizontal slices is presented.In view of the upper-bound limit analysis theory, the energy equation is derived and the ultimate failure mode is generated by comparing the sliding surface passing through the toe of the slope (mode A) with that below it (mode B).In addition, the influence of the number of slices on the stability coefficients in the IHSM is studied and stable value would be obtained. Compared to the original horizontal slice method (Chen’s method), the IHSM can acquire more precise results, which verifies the viability of this method. Moreover, the IHSM and numerical simulation method are applied to analyze the stability of a multi-layer slope with different slope angles and the results of the safety factor and failure mode are very close in each case. The ultimate sliding surfaces are shown to be mode B when slope angle is 10° or 20°. It illustrates that the determination of the ultimate sliding surface requires comparison of multiple failure modes, not only mode A, especially when the slope angle is less than 30°.