A series of model tests are carried out on flexible retaining walls such as cantilevered piles, continuous walls, and sheet pile walls in the foundation pit to study the deformation, failure surface, and earth pressure distribution of soils in a passive zone. The shape, displacement, and shear strain of slip failure surface of sand in a passive area are analyzed by Particle Image Velocimetry. The slip failure surface is a broken line, the upper end slides out from the top of the soil, and the lower end is close to the zero displacements of the retaining wall. With the increase of the flexural deformation and horizontal displacement of the wall, the shear strain of the soil increases, and the shear fracture zone in the upper part of the sliding surface is more prominent. Based on the broken line rupture surface in the test results, the passive area can be divided into two zones, the limit state zone and the non-limit state zone. Then the mechanical models are set up respectively. Considering soil displacement, the upper and lower soil layer's internal friction angle and wall-soil interface friction angle mobilize differently. The relationship between mechanical parameters along the retaining wall and horizontal displacement is estimated. Finally, the earth pressure distribution is obtained by using the horizontal differential layer method. The calculation results of this paper are consistent with the existing research results and the model test results in terms of earth pressure distribution. With the increase of depth, the unit earth pressure increases in the limit state zone. Still, after entering the non-limit state zone, the unit earth pressure rises to a certain extent and decreases rapidly.