The shape of sliding surface has important influence on the active earth pressure of retaining wall. A new active earth pressure formulation is proposed based on horizontal differential element method assuming retaining wall backfilled with cohesionless soils is under horizontal translation model. The assumption of formulation is that the sliding surface is curved when backfill behind a retaining wall reaches limit equilibrium and that the soil arching is considered. First, the reliability of the formulation is verified based on the compared calculation results of the formulation, model test and classical theory. Second, the influences of the different internal frictional angles of soil and wall-soil friction angles on the distribution, resultant and loading position of active earth pressure are analyzed. The results show the regions of sliding wedge obtained from curved sliding surface assumption are slightly bigger than the regions of plane assumption. The results by the proposed formulation are more consistent with model test results for retaining walls of different heights. The distribution form and loading position of active earth pressure are close to Paik’s solutions in different inner friction angles and friction angles on the soil-wall interface. However, the magnitudes of active earth pressure are greater than Paik’s solutions slightly.