Abstract: Computation method of pile spacing is in deficiency in the present design of guard piles. The computation model of soil arching under horizontal load was set up and stress analysis was carried out based on considering horizontal soil arching effect.The soil behind arch still bears part of stress when soil arch subjected to horizontal soil pressure,when soil behind arch is in limit state,the stress in different orientation meets certain relationship. The geometric condition of arch foot and the static balance condition, the formulas of pile spacing in cohesive and cohesiveless soil were derived based on the soil limit state. The suggestion value of pile spacing is zero for the concern of safety when the soil behind pile is cohesiveless. The suggestion value of pile spacing can be computed according to the equation when the soil behind pile was cohesive. The chosen shear strength of soil is important in utilizing the equation and the value of inner friction angle should not too large(≤20°).