Abstract: Based on the electron microscope scanning and the test of grain-size with high precision, the changes of grainsize composition before and after the treatment of dynamic consolidation are discussed with the subgrade loess of Xi'an-Yumenkou express way,and its effect on loess collapsibility is also analyzed. It could be concluded that dynamic consolidation could destroy the original grain-framework of loess, the surface cementation of conglomeration peeled, grain become thinner and filled in pores which made pores significantly reduced; soil structure become compacted and loess-collapsibility become weaker or even disappear. This study reveals the mechanism of loess-collapsibility treated with dynamic consolidation.