Abstract: The application conditions of the formula for the limit lateral pressure calculation based on the plastic deformation discussed. It is found that the plastic deformation is not applicable when the pile spacing is too small. The actual stabilizing force is related with the displacement of a slope, that is , it is associated with the safety factor of the slope. a formula for the lateral pressure of anti-slide piles with small spacing is established. Combined with the simplified Bishop's method, the formula for calculating the safety factor of slopes reinforced by anti-slide piles with small spacing is derived. The slope of a pumping station with different pile spacins is calculated. The results show that for the slope reinforced by piles with small spacing, the proposed method is reasonable.