The stability of slopes and the position determination of sliding surfaces are always the hot spots in geotechnical engineering. The rotational failure mechanism in form of the logarithmic spiral is a potential slip surface of homogeneous slope. The slope with a weak interlayer is a common problem encountered in engineering practice, where the instability occurs easily and a loss of the life and property may happen. However, there is a lack of the relevant studies on destabilization and destructive mechanism of this type of damage. In this paper, based on the upper-bound limit analysis, the failure mechanism of translational movements is adopted to investigate the stability of slopes with weak interlayer under static or seismic load and the effects of the position of a single pile and the pile spacing on the stability of slopes is compared. A notable agreement between the results obtained from the proposed failure mechanism and the previous literature is observed for the static load. Under the seismic load, there is a failure slip extending from the top of the slope towards the outside. The mid-upper position of the slope is the most effective one for a pile. The factor of safety of the slope decreases with the increase of the earthquake acceleration and the pile spacing, and the failure slip extends along the weak interlayer.