Abstract: After spraying 10 mmol/L betaine on the foliar pretreatment for 24 h, 100 mmol/L mannitol is injected into the root to simulate osmotic stress. The physiological indexes such as the MDA content, antioxidant enzyme activity, osmotic adjustment substance content, photosynthetic characteristics of leaves and roots of Brazil banana after stress treatment for different times are measured. The mechanism of exogenous betaine on the growth and physiology of Brazil banana under osmotic stress was analyzed. The research results are as follows: Exogenous betaine significantly decrease MDA accumulation in leaves and roots of Brazil banana seedlings under osmotic stress, and significantly increase the activities of SOD, POD and CAT in leaves and roots of Brazil banana seedlings and significantly increase the synthetic accumulation of soluble sugar and soluble protein. The chlorophyll content and carotenoid content of Brazil banana increase significantly, slow down the chlorophyll and carotenoid content under osmotic stress, significantly increase chlorophyll fluorescence parameters, and slow the changes of chlorophyll fluorescence parameters such as F_v/F_m, Y_II, ETR, Y_NPQ and Y_NO under osmotic stress. In summary, exogenous betaine can effectively alleviate Brazil banana osmotic stress and improve Brazil banana stress resistance.