Abstract: Microstructure plays a key role in soil behavior. Loess is a structural soil, and the study of microstructure is crucial for understanding the trigger mechanism of loess geologic hazards. In order to investigate the microstructure evolution of loess under different actions, a series of experiments including compression, wetting, and unconfined drying were conducted on compacted loess. Microstructure was studied using Mercury Intrusion Porosimetry(MIP)tests and Scanning Electron Microscope(SEM)analysis. The evolution of microstructure was discussed in terms of the effective stress of the soil. The settlement mechanism of loess-filled embankments was further revealed through the study of soil microstructure. The experimental results show that during compaction in loess, three different pore structures are formed: inter-aggregate pores, intra-aggregate pores, and micropores. An increase in as-compact dry density lead to a decrease in inter-aggregate pores, while an increase in as-compact saturation degree improved the formation of inter-aggregate pores. The as-compact state had no effect on micropore distribution. Unconfined wetting had a limited effect on pore size distribution but weakened bonding between particles. Due to the reduction of inter-aggregate pores, loess exhibited obvious collapse upon soaking under constant stress. Unconfined drying caused the shrinkage of aggregates, which further induced an increase in inter-aggregate pores and micropores, and a reduction in intra-aggregate pores. The volume change of loess during 1-D compression was caused by a decrease in inter-aggregate pore volume upon compression. In summary, the re-arrangement of microstructure is induced by changes in effective stress in unsaturated soil. However, the three levels of pore structures show different responses to variations in net stress and suction. Net stress has a greater influence on inter-aggregate pores, while changes in suction affect all three pore structures. The results of the microstructure study indicate that the quantity of inter-aggregate pores depends on the compaction degree in the field. Controlling the compaction degree can help reduce the settlement of compacted loess fill during and after construction.