The effects of bentonite addition on accumulation of organic carbon in loessal soil were investigated through a potting experiment, in which two different treatment modes including single application of different organic materials and combined application of bentonite and organic materials were set. The results showed that: 1) Compared with CK, straw returned to field (S), application of organic fertilizer (O), combined application of straw + organic fertilizer (SO), and woody peat + bio-stimulant (WE) treatments could significantly increase the soil organic carbon content by 14.0%, 30.5%, 23.9% and 30.8%, respectively, and the increase of organic carbon could be further increased to 14.8% (BS), 35.1% (BO), 26.2% (BSO) and 31.8% (BWE) after 0.2% bentonite (B) was applied. 2) The bentonite added straw returning to field (BS) and straw returned to field with organic fertilizer (BSO) treatments were more effective than CK treatment in increasing the mass fraction of large aggregates (>0.25 mm) by 7.8% and 8.3% respectively, and thus increasing the mean weight diameter (MWD) and geometric mean diameter (GWD) of soil aggregates, and enhancing the physical protection of organic carbon. 3) The addition of bentonite could enhance the content of mineral-associated organic carbon (MAOC), which was 20.6%, 28.2%, 9.4%, 9.6% and 1.4% higher in B, BS, BO, BSO and BWE treatments compared with CK, S, O, SO and WE treatments, respectively, And the content of particulate organic carbon (POC) could be enhanced by the application of organic fertilizer, of which the BO and BSO treatments could enhance the content of particulate organic carbon (POC). The BO and BSO treatments enhanced the POC content by 16.9% and 11.7% over the O and SO treatment, respectively. 4) The addition of bentonite could improve the soil carbon sequestration rate by increasing soil mineral binding carbon saturation capacity and reduced soil carbon saturation deficit. In addition, random forest analysis found that calcium carbonate is the main driver of organic carbon saturation deficit in alkaline calcareous soils, and the addition of bentonite is beneficial to alleviate the negative effect of calcium carbonate on organic matter accumulation. In conclusion, the addition of bentonite can improve soil structure and promote the accumulation of soil organic carbon in arid and semi-arid regions.