Abstract: The mechanism of "interaction effect" raised in porous material filled thin-walled structure is studied, and quantitative partition in energy absorption is reached with the proposed model in this paper. Taking aluminum foam filled hat section as example, we found three characteristic regions in the crushed foam filler, i.e., densified region, extremely densified region and undeformed region. An analytical model according to the experimental observation was built to find the volume reduction and volumetric strain in each region of the crushed porous material filler, based on the ideally crushable assumption. Combining with the superfolding element model for thin-walled hat structures, the contribution of each component of the filled structure, i.e., hat section, porous filler; densified region and extremely densified region to the overall energy absorption was quantitatively partitioned. The study shows that little increase in energy absorption is found in the thin-walled structure, while the augmentation in porous material filler is 40% or so. The extremely densified region accounts for mainly to the interaction effect.