Large-scale compression test for the improved filler from blast furnace slag by the large direct shear apparatus was carried out, and the confining deformation modulus of different stages in the loading, unloading and reloading was obtained. The research results show that water content has little influence on the compress- ibility of the filler, as there are few fine particles in the filler and the coarse particles have good water stability. Compression in the first stage of loading mainly results from smash of edges and corners and breaking off of long flat shape particles. The settlement calculation of the foundation should refer to the deformation modulus at the stage of reloading when the foundation is constructed by dynamic consolidation. The content of fine particles should be increased for the improved filler from blast furnace slag to reduce stress concentration of coarse parti- cles and make the best use of coherence of the flyash.