In order to study the critical seismic coefficient and stability factor in different directions under the distribution of seismic forces, this paper, based on the traditional Sarma, assumes that non-horizontal seismic force, at any given positions of seismic force, and try to derive the formula. Through the analysis, on the one hand, the direction of seismic forces represented by the minimum value is not necessarily pointing the outside horizontal direction. On the other hand, the corrected minimum critical seismic coefficient represents the authentic status that the smallest seismic force can be withstood by slope. Last but not the least, there is no sharp contrast between the calculated slope stability factor and the original slope stability factor. That is to say, when assuming the direction of seismic force, there will be no inaccuracy of assuming any position of the seismic force to calculate the valid slope stability factor.