The invention belongs to the technical field related to geotechnical engineering model test methods, and discloses a model test device and method for researching decompression performance of a flexible cushion layer behind a retaining wall. The model test device comprises a model casing, a rigid face plate, the flexible cushion layer, a movable baffle, a vertical loading system and a testing system; the rigid face plate is arranged at the front end of the model casing and rotationally connected to the bottom of the model casing, the flexible cushion layer is arranged on the inner side of the rigid face plate, and the movable baffle is detachably connected to the model casing; the vertical loading system is arranged on the outer side of the model casing, and the testing system is adjacent to the rigid face plate; the earth filling width is changed by changing the mounting position of the movable baffle; and the included angle between the rigid face plate and the bottom of the model casing is changed through rotation of the rigid face plate relative to the model casing, and then retaining walls at different inclination angles can be simulated. The applicability is good, the structureis simple, and processing is easy.