The sudden drop in temperature caused by cold waves is a common form of low-temperature stress. This study aims to investigate the effect of exogenous proline and melatonin on enhancing the adaptive capacity of Callistemon rigidus under cold wave conditions and to preliminarily explain the relationship between stomatal density, length, and photosynthetic capacity. Four groups were set up with the application of 500 ml of double-distilled water control group (CK), 100 μmol/Lmelatonin (M), 100 mg/L proline (P), and 100 mg/L proline + 100 μmol/L melatonin (MP), and an open field experiment was conducted at the onset of the first cold spell in 2022. Results show that: (1) The cold wave caused a decrease in stomatal length (SL) and an increase in stomatal density (SD) of Callistemon rigidus, with a reduction in leaf net photosynthetic rate (Pn) and PSⅡ maximum photochemical efficiency (Fv/Fm), and an increase in malondialdehyde (MDA) content and leaf damage degree (II). (2) SD was significantly negatively correlated with stomatal conductance (Cond) and significantly positively correlated with chlorophyll content, while SL was significantly positively correlated with photochemical quenching coefficient (qP) and proline content. There was a significant negative correlation between SD and SL. (3) The application of exogenous substances maintained stomatal morphology, increased Pn and Fv/Fm, and reduced MDA content and II, with the effectiveness ranked as MP > P > M. The application of proline and melatonin maintained the stomatal structure of Callistemon rigidus, increased the photosynthetic rate, reduced MDA accumulation, and enhanced plant stress resistance; among them, the combination of 100 mg/L proline + 100 μmol/L had the optimal effect.