Agroathelia rolfsii, causative agent of southern blight, is a soil-borne pathogen responsible for significant economic losses of Aconitum carmichaelii Debeaux. Biocontrol strategies based on beneficial microorganisms show broad prospects in sustainable plant disease management. A rhizobacterium Soil-4-2, isolated from rhizospheric soil of healthy A. carmichaelii plants, exhibited significant antagonistic activity against A. rolfsii both in vitro and on A. carmichaelii root slices. A two-year field trial (2023-2024) demonstrated that Soil-4-2 significantly reduced southern blight incidence of A. carmichaelii with biocontrol efficiency reaching 89.9 % in 2023 and 68.0 % in 2024, respectively. Furthermore, Soil-4-2 significantly promoted fresh and dry weights of stems, main roots, and lateral roots of A. carmichaelii compared to control treatment. Through whole-genome sequencing, average nucleotide identity (ANI) analysis, and multilocus sequence analysis (MLSA) of concatenated 16S rRNA, gyrA, rpoB, and recA sequences, Soil-4-2 was identified as Bacillus velezensis. The genome of Soil-4-2 harbored 14 biosynthetic gene clusters (BGCs) involved in secondary metabolites production with potential antimicrobial activity. Soil-4-2 demonstrated the ability to produce cellulase, protease, and indole-3-acetic acid (IAA). Furthermore, both cell-free culture filtrate and volatile organic compounds (VOCs) from Soil-4-2 exhibited strong in vitro inhibitory activity against A. rolfsii. Additionally, VOCs of Soil-4-2 caused disruption of the hyphal cell wall and membrane, and accumulation of reactive oxygen species (ROS) in A. rolfsii. Two main VOCs produced by Soil-4-2 were identified by solid-phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS) analysis, among which 2,4-di-tert-butylphenol (2,4-DTBP) demonstrated strong antifungal activity against A. rolfsii in a concentration-dependent manner (with EC50 value of 0.08 mmol/L). Altogether, this study proved that B. velezensis Soil-4-2 represented promising biocontrol candidate against southern blight in A. carmichaelii.