Soil arching between anti-sliding piles is the interaction of horizontal soil arch and vertical friction arch, with distinct three-dimensional characteristics. Numerical model by the particle flow code PFC^3D was developed and a series of measurement balls to numerically monitor stress variations at different positions behind the anti-slide piles were set. Combined with the displacement of numerical model balls, we analyzed the formation process and evolution pattern of the soil arching effect were analyzed. Also the thickness of soil arch had been studied and finally we proposed a new method to determine the its thickness was proposed. The study shows that the soil arch initiates s from the free face near the bottom of pile and develops to the inner and upper part of the soil, and the failure process of arch expands from the bottom of pile to the top; The thickness of soil arch varies with depth, increasing first and then decreasing along the bottom of pile to the top; The thickness of soil arch varies with time. As the loading stage proceeds, the thickness of soil arch increases first and then decreases until the soil arch is destroyed.