Abstract: High salinity in coastal saline soils induces oxidative stress, thereby limiting plant carbon sequestration. Thus, mitigating this process is crucial for enhancing the carbon sink function of the coastal saline soil-plant system. This study investigates the combined effects of biochar and arbuscular mycorrhizal fungi (AMF) on alleviating salt stress and enhancing the plant antioxidant defense system. Pot experiments and statistical modeling were employed to quantify the regulatory pathways of the biochar-AMF interaction on the soil-plant system under saline conditions. The results indicate that: (1) The combined application of biochar and AMF was more effective at a salinity of 3.80‰ than at 6.10‰. At 3.80‰ salinity, it reduced soil salt concentration by 23.75% and increased the activities of dehydrogenase, peroxidase, and catalase by 2.61-, 2.13-, and 2.40-fold, respectively, compared to the control. These enzymes contribute to the reduction of salt ion activity and the decomposition of peroxides, thereby protecting plants from oxidative damage under salt stress. (2) The biochar-AMF interaction also raised soluble sugar content by 38.96% and lowered reactive oxygen species levels, resulting in reductions of 28.48% in hydroxyl radicals, 61.36% in superoxide anions, and 79.35% in hydrogen peroxide. (3) Partial least squares path modeling further reveals that biochar mitigated soil oxidative stress and enhanced the activity of soil redox-related enzymes by improving microbial community structure. In contrast, by regulating osmotic substances, AMF activated plant antioxidant defenses, reduced reactive oxygen species accumulation and promoted plant growth. These findings provide a theoretical basis for enhancing carbon sequestration in plants.