Abstract: Based on the balance of forces acting on interior earth fill of a counterfort structure, this paper proposes a theoretical calculation method of earth pressure on each member （including wall slab, counterfort and bottom slab） when the earth fill behind structure is in the condition of active critical equilibrium. Bacausa ths friction effect of counterfort, earth pressure on the wall slab, in fact,is entirely different from classic theory（Cou-lomb or Rankine） which is used for calculating stability of structure. The friction effect of counterfort results in a remarkable decrease of the horizontal earth pressure on the wall slab. Meanwhile, the vertical earth pressure on the bottom slab is also less than the overburden pressure. The distribution of earth pressure intensity on the wall slab with depth is non-linear. Results of reducing pressure on wall slab and bottom slab depend oa the mobilized friction force of counterfort face and the distance between two counterforts. Calculation for some data of model tests and field observations indicates the theoretical values calculated by the method in this paper are in general agreement with the measured values on models. In the Steel Wharf of Huang-pu New Port, as an example, the horizontal earth pressure on the wall slab is less about 40% than the classic active earth pressure and the vertical one on the bottom slab about 30% than the overburden pressure when friction between embankment fill and concrete wall is considered as δ=φ/3