In order to prevent the uneven settlement of the road caused by the collapsibility of loess，the single-line method was used to determine the amount of collapsible deformation of strong and weak collapsible loess in Xining area under different pressures. The internal structure characteristics of loess were studied by scanning electron microscopy and mercury intrusion meter. The correlation between the internal structure of the loess and the macroscopic collapse deformation was discussed，so that the height of the subgrade was rationally designed. The results show that:(1)with the increase of pressure，the collapsible deformation of strong and weak collapsible loess shows three stages of “significant-smooth stable”，and the strong collapsible loess is weaker than the weak collapsible loess. The slope of change growth is steeper and the development speed is faster;(2)compared with weak collapsible loess and strong collapsible loess，it is found that the particles before and after water soaking are loose and easy to be compacted，the pores are also filled with fine aggregates and the connectivity is poor，and the connection method mainly is consisted of indirect contact;(3)compared strong collapsible loess with weak collapsible loess，it is found that the proportion of pore area before and after water soaking is decreased greatly，the pore order is poor，and the pore circularity is reduced. The particles are easily crushed and the degree of grouping is weak，the amount of mercury in the pores is large，and the content of the pore size distribution at 20 um is high. Therefore，the microstructural changes of the loess have a good correlation with the macro-collapse of the loess.