Abstract: Based on the clarification of the failure forms of different rock masses between piles, analyses on rock-arch effect of anti-slide piles and determination method of rational pile spacing in slope engineering project were conducted in the present method. In this paper, the formation and evolution of arching effect of soft rock were illuminated, and the comparative analysis for characteristics of rock and soil arches was carried out based on the internal force, failure mode and position in arch axis. Using the above-mentioned analyses, the catenary arch-axis equation was proposed, and the two-hinged arch theory was adopted to calculate the internal forces of rock arch. Moreover, the expression of arch-axis coefficient m concerning pile spacing l and arch height f was established using the ultimate equilibrium condition in mid-span rock mass. Moreover, the MATLAB software was adopted to assess the rational pile spacing. For practical purposes, the rational pile spacing was given to fully develop the rock-arch effect and meet the requirements of economy. The data of engineering example was used to verify the rationality of the present calculation method. The present method can be referenced for similar engineering designs.