Abstract: There have been various hypotheses and theories concerning the collapsing deformation of loess soils, e.g. capillary effect, salt dissolution assumption, clay-colloidal shortage hypothesis, water-film wedge action and under-compaction theory. They partly explain the collapsing phenomenon in terms of physics, physico-chemical or geological mechanisms. However, these hypotheses or theories didn’ t fully expound the intrinsic nature of collapsing deformation phenomenon of loess soils.In this paper the collapsing deformation of loess soils is analyzed and a structure theory for collapsing loess soils is proposed on the basis of analyses of all these hypotheses or theories, and then all phenomena and mechanisms of collapsing deformation of loess soils are clarified by the structure theory.