Twoyearold potted Malus baccata seedlings was used as experiment materials. Normal watering was used as the control CK, different concentrations of ALA (0/25/50/75 mg/L) were sprayed under drought stress. The physiological indexes of drought resistance and photosynthetic fluorescence parameters in leaves at different periods of continuous stress (day 0/4/8/12) were measured. The effects of ALA on chlorophyll fluorescence, physiological and biochemical characteristics of chlorophyll fluorescence and its regulation mechanism under drought stress were studied. The results showed that: (1) exogenous ALA increased the contents of osmolytes (SS, SP, Pro) and promoted the activities of antioxidant enzymes (SOD, POD and APX), while reduced the accumulation of malondialdehyde (MDA) in M. baccata under continuous drought stress conditions; (2) exogenous ALA increased the stomatal opening in leaves of M. baccata, promoted the synthesis of chlorophyll, and significantly alleviated the decrease of the maximum photochemical rate of chlorophyll fluorescence parameters (F_v/F_m), the actual photochemical efficiency (Y_Ⅱ), the photochemical quenching coefficient (qP) and the photosynthetic electron transport rate (ETR), as well as the increase of the quantum yield of PSⅡ regulated energy dissipation (Y_NPQ), quantum yield of nonregulated energy dissipation (Y_NO) and nonphotochemical quenching coefficient (NPQ/4) induced by drought. The results showed that the application of ALA could relieve the damage of the physiological and metabolic function of M. baccata under drought stress. However, there was a difference in response to different concentrations of ALA in M. baccata, of which 75 mg/L ALA had the most significant effect.