Abstract: The behavior of undisturbed loess in the case of moistening or demoistening is investigated using double-line uniaxial compression test in this paper. The collapsible deformation rules of collapsible loess are examined and analyzed, during the growth process of humidity and pressure. And the microstructure of the samples are tested using SEM technique. Furthermore the microstructure of samples is analyzed with quantitative method and fractal theory. The results are shown the followings:(1)Rigid node failure of trellis pores can cause the loess collapsibility.(2)Water content of loess has a marked effect on its collapsible deformation. When pressure is some value, its exceeding collapsible probability can increase as moisture content increases. Similarly, the test data show that for loess with constant water content, its exceeding collapsible probability can increase gradually as pressure increases.(3)The average number of pore of sample obviously increases and the pore area decreases after the sample is collapsed. The collapse is a process that small pore increases, fractal dimension decreases, and complexity of the pore increases. The concept of the exceeding collapsible coefficient is put forward. An analysis is made on some issues about study of hydrocompaction from the view point of unsaturated soil mechanics. Finally a case study is given. It reflects the properties of collapsible deformation under different factors.