This paper presents a study of the load-deformation behaviour of geocell-reinforced subballast under cyclic loads using laboratory tests and discrete element method (DEM). A series of large-scale laboratory tests with and without geocell inclusions are carried out using a track process simulation apparatus to study the beneficial effect of the geocells in decreasing the lateral and vertical deformations of railway subballast. Numerical studies conducted in the DEM can capture the reinforcement effect of geocells, considering micromechanical analysis subjected to a given frequency and load cycles, the predicted load-settlement response of the subballast with and without geocell agrees well with those measured experimentally. This finding indicates that the proposed DEM model is able to capture the deformation behaviour of the subballast reinforced by the geocells. These observations clearly emphasise the beneficial effects of the geocell in decreasing the deformation of subballast from a micromechanical perspective.