In the context of global climate change, it is an indisputable fact that elevated temperatures will affect plants. An experiment was conducted to study the variations of the physiological indexes in leaves of Monarda didyma L. under temperature-increasing treatment (3 ℃ higher than control) in open-top chambers (OTCs). The results showed that: (1) At 7th day of experiment treatment, compared with control, net photosynthetic rate (P_n) and transpiration rate (T_r) of Monarda didyma L. leaves extremely significant increased by 43.8% and 73.6% (P<0.01); while at 14th day of treatment, compared with control, PSII electron transport quantum yield (Phi_(PSII)) and photochemical quenching coefficient (q_P) of Monarda didymaL leaves significantly decreased by19.6% and 17.1% (P<0.05), the non-photochemical quenching coefficient (qN) significantly increased by 17.0% (P<0.05); at 14th day of treatment, compared with control, the chlorophyll content and carotenoid content significantly decreased by 10.0% and 14.6% (P<0.05). (2) At 7th day of experiment treatment compared with control, the membrane lipid peroxidation MDA content of Monarda didyma L. leaves significantly decreased by 25.2% (P<0.05); at 14th day of treatment, MDA content and superoxide radical production rate of Monarda didyma L. leaves increased by 12.5% and 15.5% (P<0.05), while the soluble protein content was significantly decreased by 19.9% (P<0.05). Therefore, temperature enhancement at the 7th day of treatment has a positive effect on the plant and promotes the photosynthesis and growth of the plant leaves, but at the 14th day of treatment, the warming habitat turns into a stress, which may has an adverse effect on plant growth, physiological and biochemical metabolism, and medicinal and edible values.