Abstract: Collapsibility is the unique feature of loess. Problems induced by collapse of loess include deformation of foundation, subsurface erosion, and slope failure. A case study of loess collapse at Heifangtai of Yongjing County in Gansu Province was analyzed in this paper. There exists a notable difference between the calculated result with the recommended equation in the national standard and the real collapse value. The calculated value of collapse is less than 50% of the motivated data. The definition of collapsible loess, the calculation method and the related issues on the assessment of loess collapse were discussed, and the results show that collapsible loess is typical unsaturated loess, and water content and structure are key factors affecting collapsibility of loess. Only by adopting the theory of mechanics of unsaturated soil and employing the parameter representing both the water content and the structure can the research on loess collapse realize the change from "why collapse" to "how collapse" so as to achieve the purpose of better serving engineering construction.