In order to provide theoretical basis for improvement of apple production under rainshelter cultivation, we evaluated the effect of 5aminolevulinic acid (ALA) on low light tolerance of apple trees in this study. Specifically, the effects of 10 mg·L^-1 ALA on reactive oxygen metabolism and fast chlorophyll fluorescence characteristics of apple trees under low light conditions were investigated by root ALA application. Two apple cultivars, ‘Runtai 2’ and ‘Zhengyou 3’, were used, and three light conditions including normal natural condition (Control), light shading stress (LS), and severe shading stress (SS), were designed in our experiment. The main results were as follows: (1) Compared to the control, shading stress significantly decreased superoxide dismutase (SOD) and peroxidase (POD) activities, and increased the production rates of super anion (O^-·₂) and hydrogen dioxide (H₂O₂) and malondialdehyde (MDA) content in both apple cultivars. More increases in MDA content and the production rates of O^-·₂and H₂O₂ were found in ‘Zhengyou 3’. ALA significantly improved antioxidant enzyme activities, and reduced the production rates of O^-·₂and H₂O₂ and MDA content. The antioxidant capacity of ‘Zhengyou 3’ was improved more significantly by ALA than that in ‘Runtai 2’. (2) Chlorophyll contents and chlorophyll b/a ratio in apple trees increased under shading stress. ALA further promoted chlorophyll contents and chlorophyll b/a ratio in apple trees under shading stress. (3) Chlorophyll fluorescence parameter V_J, M_o, and DI_o/RC significantly increased, and ψ_o, ψE_o, ψR_o, PI_ABS, PI_CS, and PI_total dramatically decreased under shading stress. ALA inhibited changes of these chlorophyll fluorescence parameters under shading stress, even made them recover to the control levels. These results suggested that oxidative damage occurred in apple trees under low light conditions due to their low antioxidant capacities, and antioxidant capacity in ‘Zhengyou 3’ was lower than that in ‘Runtai 2’. ALA enhanced antioxidant capacity of apple trees, decreased low lightinduced oxidative damage, and improved ability of trapping and utilization of light energy, photosynthetic electron transfer ability, light use efficiency, and photosynthetic performance in apple trees under shading stress. Taken together, our results demonstrated that ALA improves low light tolerance of apple trees.