A global trend of woody plant expansion is an important ecological issue in arid and semi-arid regions. Knowledge gaps remain regarding alterations in above- and below-ground vegetation structures, and the importance of their regulation on soil organic carbon (SOC) storage during woody plant expansion process. In this study, typical shrub-covered grasslands were selected along the mean annual precipitation (MAP) gradient to investigate the regulatory effects of above- and below-ground vegetation structures on SOC storage, with shrubuncovered plots used as controls. The results revealed that, woody plant expansion significantly increased root density, surface area density, node density, and branch density. "Bimodal trends" in the root surface area and node density were observed, with peaks at 136 mm and 236 mm MAPs. Woody plant expansion significantly reduced the evenness and Simpson's index in the 132-186 mm MAPs. Woody plant expansion significantly reduced total organic carbon (TOC) storage by 28.52 %, particulate organic carbon (POC) storage by 15.41 %, and mineral-associated organic carbon (MAOC) storage by 22.72 %, and increased dissolved organic carbon (DOC) storage by 30.38 %. With the decreasing MAP gradient, the TOC and MAOC storage in the shrub-covered grasslands tended to decrease, whereas the POC and DOC storage tended to first increase then decrease. During the woody plant expansion process, the key root structure influencing SOC shifted from roots >3 mm to roots <3 mm in diameter. In shrub-covered grasslands, roots with diameters of 1-3 mm were the primary contributors to POC storage, whereas those with diameters <1 mm were the primary contributors to DOC. The main factors influencing SOC storage changed from the combined above- and below-ground structures to the aboveground structure, which explained 84.7 % of the variation. The main factors influencing POC storage changed from climatic factors to belowground structural factors. The main factors influencing DOC storage changed from climatic factors to a combination of above-ground structural and climatic factors. Climatic factors were the main factors influencing TOC and MAOC storage.