Tree peony (Paeonia section Moutan DC.) is an excellent ornamental plant, of which Paeonia ostii (P. ostii) has a high oil value. It is widely cultivated in China, but severe drought affects its growth. In this study, the effects of exogenous calcium on drought-induced damage of P. ostii were studied. The results showed that under drought stress, leaf water content showed a downward trend, while reactive oxygen species (ROS), relative electrical conductivity (REC), proline (Pro) content, and related antioxidant enzymes increased significantly. Spraying CaCl2 could effectively slow leaf wilting and water loss, induced an increase in enzyme activity of the antioxidant enzyme system, and reduced the accumulation of ROS caused by drought stress. Simultaneously, REC and Pro content could be alleviated, and the degree of cell membrane damage could be reduced. In addition, CaCl2 improved photosynthetic characteristics and chlorophyll fluorescence parameters. These results indicated that CaCl2 reduced the harmful effects of drought stress on the growth of P. ostii by regulating infiltration, activating photosynthesis, and enhancing the antioxidant system. These findings suggested that CaCl2 can be used to manage drought stress in P. ostii cultivation.