Soil structure plays an important role in organic carbon (OC) sequestration, thereby influencing soil fertility and changes in global climate. However, aggregate OC chemical structure changes due to long-term return of straw in oasis farmland of arid northwest China remains unclear. This study conducted 0-, 5-, 10-, 15-, and 20-year straw returning experiments during which three soil components where measured: (1) the functional carbon (C) pool and macroaggregates; (2) microaggregates and silt + clay; (3) the chemical structure of soil OC (SOC). The results demonstrated that in comparison with the control, straw return increased SOC, particulate OC (POC), and mineral-associated OC (MAOC) by 21.90%–63.51%, 5.00%–31.00%, and 46.00%–226.00%, respectively. With increasing duration of straw return, microaggregates transitioned to macroaggregates, and percentages of soil macroaggregates under 10-year straw return increased by 20.26%, 3.39%, 4.40%, and 11.12% compared with that under 0-, 5-, 15- and 20-year straw return, respectively. Soil geometric mean diameter (GMD) and mean weight diameter (MWD) first increased and then decreased, with maximum values after 10-year straw return at 1.20 mm and 1.63 mm, respectively. Solid state 13C NMR (Nuclear Magnetic Resonance) indicated O-alkyl C to be the dominant chemical component of soil OC over different years of straw return. There were increases in aromatic C, aromaticity, and hydrophobicity up to 10-year straw return, after which they decreased. A mantel test confirmed positive correlations of the distributions of macroaggregates, microaggregates, OC of macroaggregates, and silt + clay with MWD and GMD, whereas the OC content of aggregates was positively correlated with O-OA and hydrophobicity. Long-term straw returns improved soil structure and stabilized soil OC, thereby facilitating soil sequestration of OC.