Abstract: In view of the experimental phenomenon of the structural state change of expansive soil before and after water-swelling, we put forward the viewpoint of structural expansion of expansive soil. Through proper simplification of expansive soil structure, based on particle accumulation theory, we established an idealized particle accumulation model of expansive soil structure expansion. Through the geometric analysis and mathematical derivation of five ideal stacking structures including pyramid stacking, we established the cross-scale expansion deformation equation from soil particles to soil, and discussed the influence of particle stacking structure changes before and after water-swelling on soil expansion deformation. Based on the particle packing model and expansion deformation equation of expansive soil structure expansion, we explained the structural expansion mechanism from soil particles to soil.