Abstract: To achieve coastal soft soil stabilization and agricultural waste valorization under the “dual-carbon” targets, this study proposes a novel modification strategy combining sugarcane bagasse biochar (SBB) with carbonation curing. Unconfined compressive strength tests, triaxial shear tests, and microstructural analyses (SEM, TGA, and EDS) were performed to investigate the effects of SBB content and carbonation time on cemented soil performance. Results showed that SBB content and carbonation time exhibited a significant threshold effect. The optimal performance was achieved with 3% SBB and 12 h carbonation curing, increasing compressive strength, cohesion, and internal friction angle by 26.2%, 41.5%, and 7%, respectively, compared with the untreated control. Marginal efficiency analysis confirmed that excessive SBB or carbonation reduced the enhancement efficiency. Microstructural analyses revealed that SBB improved matrix densification through pore filling, nucleation effects, and enhanced CO₂ diffusion, promoting CaCO₃ precipitation. Meanwhile, carbonation curing facilitated secondary pozzolanic reactions between active SiO₂ in SBB and calcium ions by reducing the Ca/Si ratio, further strengthening particle bonding. Carbon sequestration evaluation indicated that carbonation curing increased carbon content by 76%, reduced the Ca/Si ratio to 0.060, and enhanced relative carbon sequestration by 53%. Finally, This study not only reveals the synergistic mechanism between the SBB content and carbonation curing, but also clarifies the microscopic mechanism by which SBB synergistically improves the BBCS specimens through carbonation curing. Quadratic models were established to describe the relationships among SBB content, confining pressure, and peak deviatoric stress under standard and carbonation curing conditions, with R² values of 0.95 and 0.90, respectively. This study provides theoretical insights into low-carbon modification of coastal soft soils and sustainable utilization of agricultural waste.