To study the effects of buffer layers on the mechanical and deformation characteristics of the pile-slab wall structure supporting expansive soil slopes， the combined pile-slab wall-buffer layer ［including polystyrene foam （EPS） buffer layer and bagged gravel buffer layer］ supporting structure of a railway expansive soil slope in Dangyang， Hubei Province was taken as the object. Through a field test， the change laws of soil moisture， pile displacement， earth pressure behind the inter-pile slab， and pile and inter-pile slab bending moment of the combined supporting structure under the climate environment were analyzed. The interaction mechanism of the expansive soil slope-buffer layer-pile-slab wall combined supporting structure and its synergistic mechanical and deformation relationship were revealed under different buffer layers. The research results show that compared with the bagged gravel buffer layer， the EPS buffer layer can effectively reduce the lateral swelling pressure， with a maximum reduction rate of 69% obtained by this test. Meanwhile， the EPS buffer layer can rapidly respond to the change of swelling-shrinkage behavior of expansive soils caused by the climate environment and then dynamically improve the distribution of total lateral earth pressure behind the slab， while the bagged gravel buffer layer is relatively weak in response to the climate environment. A stable and coordinated interaction can be formed between the expansive soil slope-EPS buffer layer-pile-slab wall structure， which can reduce the influence of swelling characteristics of expansive soils on the bending moment of inter-pile slabs.