The influence of the length of prestressed reinforcement on the distribution of horizontal earth pressure and the prestress mechanism of prestressed reinforcement were studied by using scale model test and three-dimensional finite element simulation. The study showed that the value and distribution of the earth pressure on the panel were independent on the length of prestressed reinforcement. With the action of prestress, the horizontal earth pressure value of prestressed reinforced layer was large at ends and small in the middle along the direction of reinforcement. The increase range of the pressure at middle of the layer decreased with the increase of the length of the prestressed reinforcement. With the increase of prestress, the horizontal stress value of filler can exceed the vertical stress value and coincide with the large principal stress value completely. The average failure ratio coefficient of filler decreased with the increase of prestress, and the stress level of filler presented an optimal safety state for the filler under the action of prestress.