Abstract: Based on the hypothesis of the catastrophic instability of microstructure, the deformation mechanism of collapsible loess is researched further in this paper. Proposing a unified catastrophic model, which simultaneously describes the volumetric and shearing collapses, we obtain a complete constitutive relation for collapsible loess. The results of our computations and experiments indicate that the collapsing deformation of loess depends not only on the states but also on the paths of stresses, and that volumetric and shearing deformations are both influenced by the normal component p and the tangential component q of the stress on the octahedral plane. The cross phenomenon of the shear curves with various stress ratio can be regarded as one of natural characters of collapsible loess. Finally,some problems on the collapsing deformation are discussed using the proposed model.