Abstract: Based on the scanning electron microscopy (SEM) of microstructure and collapsible test of undisturbed loess, the microstructure properties, the collapsible characteristics under different pressures and water contents, and the intrinsic mechanism between microstructure and collapsibility of loess were analyzed. The results indicated: (1) with the increasing of imbedding depth, the configuration of loess varying gradually from bracketed-macropore weak-cemented texture to inlaid-micropore half-cemented texture, the void properties from bracketed-macropore void to void among grains, then to micropore void; (2) the curves of coefficient of collapse-different pressures of loess under humidification can be divided into three phases, at the initial phase of compressive deformation was the course of loess compaction of structural strength, with a small settlement and microstructure no destroyed; (3) the second phase after the first turning point s=0.015 of the curves was the primary course of collapse deformation, the microstructure varying and particles redistributed with along the intrinsic texture of loess damaged , and ratio of collapsible settlement increased; and (4) the third, the new microstructure coming into being, the collapsible settlement decreased after the point of peak value. (5) the values of the initial collapsible pressure and the collapsible force of peak value increased, and the coefficient of collapse of peak value and the range values of pressures of collapsible settlement diminished as the increasing of imbedding depth; (6) the characteristics of framework grains and void of loess were the intrinsic influencing factors of collapsibility, and the amount and properties of cement were one of mainly contributing factors for the intensity of collapsibility of loess.