Objective To investigate the effects of pretreatment methods on moisture variation and pore structure of apple slices during microwave freeze-drying.Methods Apple slices pretreated with four methods (ultrasound, blanching, freeze-thawing, and osmotic dehydration) are subjected to microwave freeze-drying. The effects of different pretreatments on drying characteristics, moisture migration, and microscopic pore structure of the samples are comparatively analyzed.Results All four pretreatment methods increase the effective moisture diffusion coefficient and reduce drying time by 11.1% to 44.4%, with osmotic dehydration requiring the shortest drying time. Low-field nuclear magnetic resonance (NMR) analysis indicates that the pretreatments accelerate free water removal, consequently enhancing internal moisture migration. Pore structure analysis reveals that the pretreatments improve the internal structure of the samples and increase porosity, thereby accelerating the drying process. Specifically, osmotic dehydration leads to smaller pore sizes, while ultrasound and freeze-thawing treatments result in similar porosity with relatively larger pore sizes. In contrast, blanching yields the highest porosity.Conclusion The four pretreatment methods improve the internal pore structure of the samples and promote moisture migration during microwave freeze-drying, thus enhancing drying efficiency.