Abstract: To screen biocontrol bacteria against tobacco black shank disease and to improve the efficacy of biocontrol bacteria, bacterial strain CY2 with excellent antagonistic activity against Phytophthora parasitica was isolated from rhizosphere soil of healthy tobacco plants. The strain CY2 was identified by morphological, physiological, biochemical and molecular biological methods and its fermentation conditions were optimized by single factor experiments. Preliminary evaluations were made on the disease control efficacy of the combined CY2 fermentation broth and oxathiapiprolin against black shank disease. The results showed that strain CY2 was identified as Paenibacillus polymyxa and its inhibition rate against P. parasitica was 82.1% in the plate confrontation test with a broad spectrum of antifungal activities. The optimal fermentation medium for strain CY2 was NB medium with soluble starch and yeast extract as carbon and nitrogen sources, respectively. Under the optimized fermentation conditions, namely initial pH 8, fermented for 36 h at 30 ℃ with shaker speed of 180 r/min and 2.0% inoculum size, the inhibition rate of the fermentation broth reached 93.5% when the bacterial amount increased to 9.2×10⁸ cfu/mL. When CY2 fermentation broth was combined with oxathiapiprolin at a 1 ∶ 2 volume ratio, there was no significant difference in the control efficacy compared to that of using oxathiapiprolin alone, with both exceeding 78% efficacy. However, the application of the combined agents could effectively reduce the amount of chemical pesticides used. These results provide an important reference for the subsequent field application of CY2 fermentation broth combined with oxathiapiprolin.