Medicinal plants harbor tremendously diverse bacterial endophytes that maintain plant growth and health. In the present study, a total of 124 culturable bacterial endophytes were isolated from healthyAconitum carmichaeliiDebx. plants. These strains were clustered into 10 genera based on full-length 16S rDNA sequences, among whichBacillusandPseudomonaswere the dominant genera. In addition,A. carmichaeliimay capture 10 potential new bacterial species based on multi-locus sequence analysis of three housekeeping genes (gyrA, rpoB, andatpD). The majority of these bacterial endophytes exhibited plant growth-promoting ability through diverse actions including the production of either indole acetic acid and siderophore or hydrolytic enzymes (glucanase, cellulose, and protease) and solubilization of phosphate or potassium. A total of 20 strains inhibited hyphal growth of fungal pathogensSclerotium rolfsiiandFusarium oxysporum in vitroon root slices ofA. carmichaeliiby the dual-culture method, among whichPseudomonassp. SWUSTb-19 showed the best antagonistic activity. Field experiment confirmed thatPseudomonassp. SWUSTb-19 significantly reduced the occurrence of southern blight and promoted plant biomass compared with non-inoculation treatment. The possible mode of actions forPseudomonassp. SWUSTb-19 to antagonize againstS. rolfsiiinvolved the production of glucanase, siderophore, lipopeptides, and antimicrobial volatile compounds. Altogether, this study revealed thatA. carmichaeliiharbored diverse plant growth-promoting bacterial endophytes, andPseudomonassp. SWUSTb-19 could be served as a potential biocontrol agent against southern blight.