Abstract: To screen synergistic additives for Bacillus velezensis Ba-0321 and improve its biocontrol efficacy and stability against tobacco Fusarium root rot caused by Fusarium oxysporum, the effects of seven exogenous additives on the growth of Ba-0321 were evaluated by turbidimetric assay. Plate antagonism tests, growth curves, and spore production dynamics were further analyzed to determine effective synergistic substances. A greenhouse pot experiment was conducted to assess the combined effects of the selected additives and Ba-0321 on tobacco growth, rhizosphere colonization, and the control of Fusarium root rot. The results showed that riboflavin (0.01%), chitosan (0.1%), and ammonium phosphate trihydrate (0.01%) significantly promoted the growth of Ba-0321 and inhibited the pathogen. Riboflavin mainly accelerated early bacterial proliferation and advanced the growth peak, while chitosan enhanced and maintained spore production in the later stage, and ammonium phosphate showed a moderate effect. Compared with the application of Ba-0321 alone, the three combined treatments increased tobacco biomass-related parameters by 3.6%–11.9% and root vigor by 5.50%–23.52%. Among them, the treatments of “riboflavin + Ba-0321” and “chitosan + Ba-0321” enhanced and maintained the colonization of Ba-0321 in rhizosphere soil and roots. The control efficacies against tobacco Fusarium root rot were 62.50% and 59.88%, respectively, which were 20.26% and 15.22% higher than that of Ba-0321 alone. The treatment of “ammonium phosphate + Ba-0321” showed a control efficacy of 56.20%, which was 8.14% higher than that of Ba-0321 alone. Therefore, riboflavin (0.01%), chitosan (0.1%), and ammonium phosphate trihydrate (0.01%) can serve as effective synergistic additives to enhance the plant growth-promoting activity and disease control efficacy of Ba-0321.