The formation mechanism of the arching effect between cantilever piles is studied according to their stress characteristics. The soil arch is classified into three types based on its position in a space coordinate system, namely, a horizontal arch, a vertical arch and a freeface arch. The master factor for distribution of unloading area between piles is the horizontal arch effect. The range of effective unloading area is defined. Based on the failure pattern of soil arches and the relationship between soil strength theory and the static equilibrium principle of soil arches, a calculation model is established by considering the presence of geostatic stress and sliding thrust force. The relation between soil strength and critical height is studied by variation of cohesion and friction angle. In addition, the impact of canonical distributions of the earth pressure to the vector height of unloading area is also discussed. Finally, the calculation model is verified by an example from real engineering work.