Abstract: As an effective land improvement measure, slope land preparation has a significant effect on soil and water conservation in sloping farmland in hilly and mountainous areas. Slope land preparation measures reduce soil erosion and improve land use efficiency by improving soil structure and water holding capacity. However, the existing research mainly focuses on the influence of slope land preparation on soil macroscopic characteristics, and the specific mechanism of how slope land preparation measures affect soil water holding capacity by regulating soil microstructure, especially soil aggregate structure and its related functional groups is still not very clear. Therefore, from the microscopic point of view of soil, this study focused on the effects of different slope land preparation measures on soil aggregate structure, functional groups and their quantitative relationship with soil water retention capacity. The results showed that different slope land preparation measures played a significant role in improving soil aggregate structure. Compared with slope farmland (control group), slope land preparation measures effectively changed the composition and structure of soil aggregates, weakened the cementation between soil aggregates, and then formed a small and complex aggregate pore structure. Especially in the formation of mosaic pores, slope land preparation measures can significantly improve the spatial uniformity and structural stability of soil aggregate pores. In addition, the gradation composition of soil aggregates was optimized, which improved the water permeability and retention of soil. Under the influence of slope land preparation measures, the content of hydrophilic functional groups in the soil also changed. Specifically, the relative content of C—OH groups in hydrophilic functional groups Studies have shown that the increase of C—OH groups is closely related to the change of soil aggregate porosity, which provides a more favorable micro-environment for the maintenance of soil moisture. The effects of different slope land preparation measures on soil water retention capacity were further analyzed. The results showed that the field soil maximum water holding capacity (27.06 mm), capillary water holding capacity (25.42 mm) and field water holding capacity (22.57 mm) of the horizontal platform were significantly higher than those of the reverse slope platform and slope farmland, indicating that the soil preparation measures of the horizontal platform can significantly improve the soil water retention capacity. Based on the comprehensive soil water holding characteristics, it was found that the comprehensive soil water holding capacity from large to small was: horizontal platform (75.05), reverse slope platform (57.20) and slope farmland (53.49). This result further proves the important role of slope land preparation measures in improving soil moisture retention characteristics. Comprehensive analysis showed that the fractal dimension of hydrophilic functional groups (especially C—OH groups) and soil aggregate pores were the main factors affecting soil water holding capacity. Slope land preparation measures enhance the water storage capacity of soil by regulating the pore structure of soil aggregates, thus effectively improving the water retention performance of soil. Compared with slope farmland, slope land preparation not only improves the aggregate structure of soil, but also improves the water retention capacity of soil at a deeper level, which is of great significance for optimizing the utilization of water and soil resources in slope farmland in hilly and mountainous areas. The research results provide a scientific basis for further optimizing the management and utilization of slope farmland, and have important guiding significance for improving the sustainable development level of agriculture in mountainous areas increased significantly, which played a key role in the improvement of soil moisture retention.