Abstract: Shearing strength parameters of slide surface are often determined by “back calculation method”. Analysis shows that, for the landslide of situated deformation condition, its sliding force can directly be estimated based on stability coefficient in present and safety coefficient in design. The sliding force equals to the product of distributive downslide force and the relative ratio between the stability and safety coefficients. Further, the suit conditions of the method in every design environmnents and every typical slide surface shapes are discussed. This is a simple method of estimating landslide slide force.