ABSTRACT The undisturbed and remoulded loess samples were subjected to the unconsolidated‐undrained triaxial compression test, imaged using scanning electron microscopy before and after the triaxial compression test and analysed with the image‐processing software Image‐Pro Plus to extract pore characteristics from the loess images. The experimental results show that the undisturbed loess has a metastable open structure, that aggregates serve as the main structural units of the loess skeleton and that these basic units are connected by clay material and calcium carbonate. The remoulding process destroys the initial structure of the loess, making the pore distribution more uniform and the remoulded loess is more prone to instability and damage under external forces. During the triaxial compression process, the microstructural evolution of the undisturbed and remoulded loess generally followed consistent patterns, both of which are characterised by the disruption of intergranular cementation, the collapse of aggregate structure, the filling of pores by fine particles and the overall structure becoming denser. The key difference lies in the fact that the undisturbed loess primarily featured an increase in the number of rounded pores and a reduction in the area of elongated pores, whereas remoulded loess mainly demonstrated an increase in pore number with relatively stable changes in pore area. The research reveals the microstructural evolution process of loess under compression, which is of great significance for a deeper understanding of the mechanical properties and engineering behaviour of loess. The test results can provide a basis for the study of other mechanical behaviours of loess.