Abstract: Loess Q3 is a kind of undsr-compaction soil and has collapsibility in various degrees. In this paper, the concepts of moistening deformation are defined and the mechanical model of this moistening deformation is set up. This model will contribute to understanding all the deformation characteristics of collapsible loess. The deformation process can be indicated by a stress-moistening path and a stress-moistening-deformation surface. Furthermore, the coacepts of deformation potential and collapsible potential are defined.