Abstract: To explore the potential of the synergism of Bacillus siamensis (BS) combined with Oxathiapiprolin (OX) against tobacco black shank, the biocompatibility of BS and OX was measured by plate coating method, and the inhibition effect of BS, OX and their combination on tobacco Phytophthora nicotianae were determined by the growth rate method. A pot experiment was performed to compare the effects of BS, OX and their combination on tobacco black shank disease in vivo. BS exhibited high biocompatibility with OX, and the low concentration of OX enabled BS higher growth activity. The results of in vitro toxicity test indicated that the inhibition rate of BS fermentation broth was 42.2%, while the inhibition rate reached 92.35% when combined with OX(1.25×10⁻⁴ mg/L). In the pot experiment, the 5-fold diluted BS fermentation broth combined with OX(1.25×10⁻⁴ mg/L) had control effect of 83.57%, which was 42.85% and 67.81% higher than that of BS or OX treatment, respectively, and the agronomic indexes of tobacco plants increased, so the synergistic effect was significant. The results proves that the combination of BS and OX can effectively control tobacco black shank disease, promote seedling growth, and reduce the use of OX.