In order to explore the stress resistant physiological and biochemical mechanism of exogenous 5aminolevulinic acid (5ALA) and CaCl₂ on Paeonia lactiflora cultivated in open field under high temperature stress, to provide technical basis for alleviating the high temperature injury of P. lactiflora in summer, we investigated the plasma membrane stability, antioxidant enzyme system, chlorophyll content, photosynthetic parameters and ornamental quality of P. lactiflora ‘Dafugui’ from April to July after spraying 100 mg/L 5ALA and 4% CaCl₂ on leaves in the study. The results revealed that: (1) the flowering period, flower diameter and fresh weight of P. lactiflora in 5ALA and CaCl₂ treatment groups were significantly higher than those in the control group; (2) Compared with the control, P. lactiflora treated with 5ALA and CaCl₂ could maintain higher superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities, lower superoxide anion radical ( O₂^-·) production rate and MDA content, as well as higher cell chlorophyll content, net photosynthetic rate (P_n), transpiration rate (T_r), and stomatal conductance (G_s); (3) Compared with foliar spraying CaCl₂, spraying 5ALA had a better effect on the stress resistance physiological and biochemical indexes and ornamental quality of P. lactiflora cultivated in the open field. The results showed that exogenous 5ALA and CaCl₂ treatments could increase the chlorophyll content and antioxidant capacity of P. lactiflora, enhance the photosynthetic capacity, reduce the level of membrane lipid peroxidation, and effectively improve the ornamental quality of P. lactiflora ‘Dafugui’ cultivated in open field in summer, and the improvement effect of 100 mg·L^-1 5ALA was better.