Abstract: This study explored Pseudomonas fluorescens（Pseudomonas fluorescens）5B12 as a biocontrol agent for managing gray mold disease, and developed a combined application with chemical fungicides, aiming to identify an efficient and low-toxicity chemical-biological mixture. The mycelial growth rate method was employed to determine the in vitro inhibitory activity of six chemical fungicides and P. fluorescens 5B12 against Botrytis cinerea in cucumbers. The agar plate colony counting method was used to test the biocompatibility of the fungicides with P. fluorescens 5B12 and identify the optimal combination ratio. Pot experiments were conducted to evaluate the efficacy of the mixture against the gray mold. Results indicated that boscalid, pyraclostrobin, and ipconazole exhibited strong inhibitory activity against B. cinerea, while boscalid demonstrated good biocompatibility with P. fluorescens 5B12. When pyraclostrobin (2.24 μg/mL) was combined with P. fluorescens 5B12 (2.53 × 10⁷ CFU/mL) at a volume ratio of 6:4, the inhibition rate against B. cinerea reached 64.56%, demonstrating the best synergistic effect. Pot trial results demonstrated that the efficacy of the combined treatment was significantly superior to individual treatments. In summary, the joint application of P. fluorescens 5B12 and boscalid not only improved the control efficacy against Botrytis cinerea in cucumbers but also reduced the quantity of chemical agents used, thereby providing a data basis for efficient and green control.