Abstract: In order to determine the sliding surface form of the soil backfill and calculate the earth pressure, static equilibrium equations of sliding wedge were built for cohesive soil backfill by limit equilibrium method. Then, the equation of the sliding surface and the analytical expression for the total active earth pressure were derived using the variational method. As a general result, a log-spiral sliding surface was obtained. The calculated active earth pressure values were compared with those obtained by Coulomb theory and the measured data in an engineering example. The result shows that the active earth pressure calculated by the proposed method is 5.37% larger than that calculated by Coulomb theory, and is close to the measured value. Finally, the effects on the active earth pressure of the retaining wall and the various parameters of the soil backfill with curved sliding surface were analyzed, revealing that the cohesion and internal friction angle of the soil backfill are the key factors determining the value of active earth pressure.