Abstract: To study the reasonable pile spacing of anti-slide piles, based on the soil arching effect, it was proposed a hypothesis that in the reasonable pile spacing, the soil arches at pile ends and pile sides were simultaneously activated to resist the slide forces of landslides. According to the static equilibrium condition between piles and the Mohr-Coulomb strength criterion of soil,a formula for pile spacing was deduced and checked through engineering cases. In the condition of a certain safety margin, the rationality of the formula was verified. Subsequently, the impact of variable parameters of the formula on pile spacing was analyzed. Eventually, the rationality of the formula was further verified by three-dimensional numerical simulations. The results indicated that in the condition of reasonable pile spacing, the spacing linearly increased with the cohesion of soil, front and side widths and increased with internal friction angles of soil as a quadratic function, and simultaneously, it dropped sharply as a quadratic function with the increase in uniform thrust of landslides. The three-dimensional numerical simulations demonstrated the earth arch effect decreased constantly with the increase in pile spacing, and the anti-slide effect of anti-slide piles also decreased. With the increase in pile spacing, the displacement of soil beside piles was larger and larger, and the closer to the middle section of earth arches, the greater the displacement was. However, the displacement of soil at pile ends was less influenced by pile spacing and changed slightly away from anti-slide piles compared to that closer to anti-slide piles.