Biochar and beneficial microorganisms (BM) is considered promising soil amendment for saline-alkali amelioration and soil carbon storage.However, the effects of biochar combined with BM addition soil organic carbon (SOC) accumulation and microbial characteristics are less known in coastal saline-alkali soil. Herein, we investigated the SOC content and fractions, soil carbon enzyme activities, and microbial community composition in coastal saline-alkali soil, following three levels of biochar and BM addition. Compared to the control treatment, biochar and BM application effectively reduced soil salinity by 37.58–66.53% and increased soil NH₄⁺ by 9.49–121.16% and NO₃⁻ by 43.56–254.28%, respectively. Biochar integrated with BM addition significantly increased the content of SOC, soil mineral-associated organic carbon (MAOC), soil particulate organic carbon (POC), and carbon pool management index (CPMI) by 37.76–108.02%, 15.43–140.44%, 13.73–64.55%, and 81.11–154.61%, respectively, compared with CK treatment. Additionally, biochar and BM significantly enhanced the activities of soil carbon cycle enzymes, including α-1,4-glucosidase (14.54–124.45%), β-1,4-glucosidase (12.71–133.98%), and cellulose hydrolase (6.07–19.17%). Biochar and BM addition also improved the bacterial diversity and altered the microbial composition at the phylum level. The co-addition of biochar and BM improved SOC by decreasing soil salinity and, enhancing soil nutrient availability, soil carbon cycle enzymes, and microbial activity. Furthermore, the combination of 4% biochar and BM exhibited the highest MAOC/POC ratio, demonstrating the most significant impacts on enhancing SOC stability in coastal saline-alkali soil. This study highlighted that the combined use of biochar and BM could serve as a promising approach to fortify soil carbon pool content and stability in saline-alkali land.