Based on the three-dimensional upper limit analysis theory with three-dimensional horn-like rotational failure mechanism, the power balance equation was established considering slope angle of the slope crest. The software MATHEMATICA and exhaustive method with constraint optimization were used to solve the minimum of safety factor. Under different parameter c/γH, internal friction angle φ, slope crest angle αand slope inclination angle β, the slope safety factors calculated by the strength reduction method (SRM) and gravity increase method (GIM) were quantitatively compared and analyzed. The results show that when FS is equal to one, the results calculated by GIM and SRM are close to each other and approximately equal to one. When FS is greater than one, GIM is greater than SRM. When FS is less than one, SRM is greater than GIM method. For stable slopes with FS below 1.2, the errors of GIM and SRM methods are within the acceptable range.