Bacillus is a beneficial soil microorganism that enhances plant growth and stress resistance, yet its mechanism in boosting drought resistance remains unclear. This study explored its role in enhancing herbaceous peony (Paeonia lactiflora) drought resistance via gradient soil moisture treatments (T0: 75%±5%; T1: 55%±5%; T2: 35%±5%; T3: 15%±5%). Bacillus significantly improved P. lactiflora drought resistance, especially at T2 and T3 stages, alleviating drought damage by increasing photosynthetic