【Objective】This study aims to investigate the mechanisms and efficiency-enhancing pathways by which pelletized straw incorporation technology rapidly improves organic carbon content and its active fractions in sandy soils of the Horqin Sandy Land, addressing critical challenges of organic carbon pool depletion and poor water-fertilizer retention capacity.【Method】Through a controlled incubation experiment, the study established treatments with different application rates of pelletized straw: a control with no straw addition (CK), 75 t·ha-1 pelletized straw (PS75), and 150 t·ha-1pelletized straw (PS150). Additionally, the experiment included duration treatments consisting of single-year application and two consecutive years of application.【Result】 Compared to the CK treatment, pelletized straw application significantly increased soil organic carbon (SOC) and total nitrogen (TN) contents by 217.52%～749.15% and 197.78%～679.25%, respectively. With increasing application rates and duration of pelletized straw incorporation, the carbon and nitrogen retention capacity of sandy soil was significantly enhanced. The addition of pelletized straw significantly enhanced particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and labile organic carbon (LOC) contents (p<0.05). Notably, the 150 t·ha-1 pelletized straw treatment with two consecutive years of application significantly increased the proportion of POC to total SOC by 31.81% (p<0.05), suggesting a preferential accumulation of this active carbon fraction. The observed highly significant positive correlations between particulate organic carbon (POC) and total SOC content (P<0.01), as well as between labile organic carbon (LOC) and total SOC content (P<0.01), provide compelling evidence for the pivotal role of these labile carbon fractions in driving overall soil organic carbon sequestration. Pelletized straw application significantly reduced bulk density while improving water-holding capacity and porosity in sandy soil. The study found a statistically significant positive correlation (p<0.01) between the water-holding capacity of sandy soil and the duration of pelletized straw application. This indicates that the improvement mechanism of water-holding capacity in sandy soils can be attributed to the synergistic effects of physical adsorption and chemically mediated water retention derived from pelletized straw decomposition. RDA analysis identified MAOC and capillary porosity as key factors influencing soil carbon and nitrogen retention. MAOC emerged as the dominant driving factor, exhibiting the strongest explanatory power for variations in SOC, TN, and C/N ratio. Partial least squares path modeling demonstrated that the cumulative addition of pelletized straw showed a highly significant positive correlation with soil particulate organic carbon and readily oxidizable organic carbon (P< 0.01), pelletized straw application significantly promoted SOC and TN accumulation through regulating POC (p<0.05), with the carbon sequestration effect further improving soil water retention via reduced bulk density (p<0.05).In summary, the study demonstrated that particulate organic carbon (POC) served as the key mediator for carbon-nitrogen coupled stabilization in sandy soils. The establishment of this regulatory mechanism provided a theoretical foundation for carbon sequestration management in arid sandy soils.【Conclusion】The study demonstrates that pelletized straw incorporation effectively enhances sandy soil organic carbon fractions, promotes carbon-nitrogen synergistic sequestration, and improves soil physical properties, with the optimal effects achieved at 150 t·ha-1 with two consecutive years of application.