Abstract: 【Objectives】 The physiological effects of exogenous abscisic acid(ABA) on the tolerance of sweet potato seedlings variety Xushu 25 to NaCl stress were analyzed to provide a theoretical basis for enhancing the salt tolerance and improving the yield of sweet potato.【Methods】 The sweet potato seedlings of variety Xushu 25 were treated with different concentrations of NaCl, and the leaves of sweet potato seedlings exposed to NaCl with different concentrations in culture mediums were continuously sprayed by the solutions containing different concentrations of ABA for 7 d. Root amounts of seedlings were determined, and photosynthesis index was measured using a portable photosynthetic apparatus, and chlorophyll fluorescence was determined by a plant efficiency analyzer. The concentrations of Na+, K+, Ca2+ in leaves were determined by atomic absorption spectrophotometer, and the levels of MDA, proline, soluble sugar and the activity of superoxide dismutase in leaves were respectively determined by colorimetric method.【Results】 The sweet potato seedlings were lightly affected by NaCl of 50 mmol/L. With the increase of salinity, the root growth rate was decreased, while the relative conductivity and the contents of MDA, proline and soluble sugars were increased,and the activity of superoxide dismutase showed an increase trend, and then gradually decreased. The Pn, Tr, Gs, Fv/Fm, 0 and E0 values in the leaves were decreased, but those for Vk and D0 increased. Except Na+, the contents of K+, Ca2+ and the ratio of K+/Na+ were decreased. The growth and metabolism were significantly affected by higher concentration of NaCl. Application of exogenous ABA significantly abates the poison ous effect of NaCl on sweet potato seedlings, and the best effect was obtained in ABA concentration of 70 mol/L.【Conclusions】 Exogenous ABA could significantly promote the root growth of sweet potato seedlings, reduce the degree of membrane lipid oxidation, and keep higher values of Fv/Fm, 0 and E0, but lower values of Vk and D0. Threrfore, leaf spraying with 70 mol/L of ABA is recommended to alleviate the toxicity damage of sweet potato seedlings.