Abstract: 【Objective】 In order to explore the changes of peel morphology and related indexes of orah sunburn fruits, the physiological mechanism of orah sunburn was preliminarily clarified to provide a theoretical basis for the prevention and control of citrus sunburn.【Method】 Five-year-old orange-rootstock orah was as material, and normal fruits were as control. Sunburn orah fruits peels were measured by means of paraffin sectioning and scanning electron microscope. The contents of soluble protein, nutrient elements, enzyme activity, leaf transpiration rate and chlorophyll in the pericarp were detected and the law of change was analyzed.【Result】 With the aggravation of orah sunburn, the oil cells on the pericarp surface gradually expanded and ruptured to form gullies, the burn site gradually ruptured from yellow skin layer to white skin layer, and the content of soluble protein in pericarp decreased. After sunburn, leaf transpiration rate increased and chlorophyll decreased. After sun burning, the contents of K, CA, Cu and Zn in peel of orah were significantly reduced by 2.22 mg/g, 1.95 mg/g, 2.22% and 9.84%(absolute value), respectively(P<0.05). While the contents of superoxide anion(O₂^-) and malondialdehyde(MDA) increased by 85.5% and 55.2%, activities of catalase(CAT) and peroxidase (POD) and polyphenoloxidase (PPO) increased by 74.7%, 64.2% and 45.1%, respectively, activity of superoxide dismutase (SOD) decreased by 44.6%.【Conclusion】 The protective ability of orah fruit to pericarp cells decreases with the aggravation of sunburn. The ability to resist external light and heat stress decreases. Therefore, the prevention and control of citrus sunburn can be comprehensively considered from the aspects of increasing the application of water and fertilizer, supplementing the lacking nutrients, reducing the transpiration rate of leaves, protecting the pericarp and properly retaining late summer shoots.