Abstract: Biocontrol Bacillus microbial preparations can effectively control plant diseases. In this study, the fermentation broth of Bacillus atrophaeus ZY1825 strain with biocontrol potential was used as the active ingredient of the coated seed treatment agent, and the causal agent of cabbage Fusarium wilt was used as the indicator pathogen. Single factor, orthogonal, plate confrontation test, and dilution plate counting methods were used to determine the antagonistic antibacterial effect and the content of viable bacteria. Through the screening of dispersant, film-forming agent, stabilizer, antifreeze, and other additives, the optimal formula of mixed additives was determined as follows: 4.00% sodium lignosulfonate, 0.25% carboxymethyl cellulose, 1.00% bentonite, 0.50% ethylene glycol, and 2.00% scarlet dye (mass fraction). The formulation of biocontrol Bacillus seed treatment agent WS20 was as follows: 2 : 1 of the volume ratio for B. atrophaeus fermentation broth (9.28 × 10⁹ cfu/mL) to mixed additives, and 1.45 min of film-forming time. The coating uniformity was 93.06%, and the shelf life was up to 150 d. The indexes of heat storage and low-temperature stability met the relevant national standards of seed treatment suspension agents and microbial preparation. The control effect on cabbage Fusarium wilt was 64.94% when the cabbage seeds were coated with WS20 at a mass ratio of 1 : 30. The research results can provide a reference for the preparation of biocontrol Bacillus seed treatment agents and the biological control of soil-borne diseases.