Considering soil arching effects, the internal friction angle and cohesive force between pile and soil, between soil particles are generally under nonlimit state. Cohesive soil slope supported by flexible pile is studied and the potential slope sliding surface is searched and the formulas of the active earth pressure of cohesive soil under nonlimit state against flexible pile is derived via Mohr circle of stress, micro layer analysis method as well as the static equilibrium. The difference between the theory proposed and Rankine solution is studied via comparison with engineering applications. The active earth pressure calculated by proposed method in this paper is greater, while the location impacted by resultant force of the active earth pressure is higher. Besides, the ranse of potential sliding surface is smaller than that of sliding surface under Rankine limit state.