[Objective] This study aims to investigate the composition and distribution of major element oxides in sediments along the Three Gorges Reservoir (TGR), to elucidate the geochemical cycling characteristics of major elements in sediments of a mountainous river-type reservoir. [Methods] Sediments from the TGR were collected as the research object, with riverbed and suspended sediments analyzed separately for grain-size composition and major element oxide contents. Geochemical parameters including the Chemical Index of Alteration (CIA), Na/K ratio (RN/K), Si/Al ratio (Sa), dealkalization coefficient (W), and residual coefficient (Ki) were calculated to analyze variation patterns of sediment geochemistry. [Results] ① The sediments are dominated by fine particles, with average contents of silt, clay, and sand of 87.04%, 6.91%, and 6.05%, respectively. Overall, the sediments exhibit poor sorting, with skewness ranging from fine-skewed to symmetrical, and kurtosis mostly showing mesokurtic characteristics. Spatially, grain size decreases progressively from upstream to downstream, and particle sorting improves accordingly. ② The major element oxides in sediments are dominated by the non-metallic oxide SiO2, followed by the metallic oxides Al2O3, Fe2O3, and CaO, while K2O, MgO, and Na2O occur at relatively low levels. In the suspended sediments, the contents of CaO and MgO are slightly higher than those of Fe?O? and K2O, respectively. From upstream to downstream, SiO2 and Na2O contents show a decreasing trend along the reservoir, whereas Al2O3, Fe2O3, K2O, and MgO gradually increase, and CaO exhibits no clear distribution pattern. Compared with the Upper Continental Crust (UCC), the sediments are depleted in SiO2, K2O, and Na2O but enriched in Al2O3, Fe2O3, CaO, and MgO. ③ The sediments are in a moderate stage of chemical weathering under a warm and humid climate, with mean values of CIA, RN/K, Sa, W, and Ki of 73.26, 0.39, 5.25, 0.78, and 1.07 for riverbed sediments, and 70.79, 0.68, 5.68, 0.94, and 0.85 for suspended sediments, respectively. ④ Except for CaO, the contents of major element oxides in sediments show statistically significant correlations with grain-size composition(p<0.05). [Conclusion] The major element oxide contents in sediments of the TGR exhibit pronounced spatial differentiation along the reservoir, which is mainly influenced by factors such as the relative contributions of potential source materials and grain-size effects caused by hydrodynamic sorting.