Aim at the soil arch effect between piles,analyze the suitable space between the anti-slide piles. The paper deduce the formula for calculating the largest space of anti-slide pile. The control conditions is that the Mohr coulomb strength condition of soil at the most disadvantageous position at the soil arch springing and the shear stress is small then the maximum sliding resistance at each point on the pile-soil contact surface. Another control condition is that the landslide thrust act on the pile and the soil between piles is small then the flow resistance undertake by pile. The formula is clear and simple .The is formula is reasonable then calculation an engineering example, it has the value of project reference.