Considering the soil-arching effect of retaining wall backfill of rigid translational mode, the paper analyzed the influence of division differential unit on along the arc arch tangent and secant . On above basis,it obtained the formula for the active earth pressure against a rigid retaining wall by means of dichotomy to the optimum dipartition mode. Through comparing the formula with the theoretical solutions of tangent direction, secant direction and the results of measured test,the result can be goten that the active earth pressure proposed is smaller than those of tangent direction and larger than those of secant direction.The active earth pressure distribution of rigid wall backfill gotten by the method match best with the physical model test.