Abstract: 【Objective】 This study aimed to investigate the effects of salt stress on salt ion distribution and physiological characteristics of grafted watermelon seedlings, providing a theoretical reference basis for regulating the growth of watermelon under salt stress and rationally developing and utilizing underground brackish water and salinized soil. 【Method】 The grafted watermelon seedlings were used as experimental materials, and the Hoagland nutrient solution with a nutrient solution salinity of 2.5 g/L as control（CK） was set as the base, supplemented with brackish water solutions with different salinities（3.0, 3.5, 4.0, and 4.5 g/L）, to determine the fresh weight（FW）, dry weight（DW）, the relative content of chlorophyll（SPAD value）, K⁺, Na⁺, Cl^-accumulation and K⁺/Na⁺ values in different tissues, as well as various physiological indicators superoxide dismutase（SOD） and catalase（CAT） activities, proline（Pro）, and malondialdehyde（MDA） contents) of watermelon seedlings under the different treatments. Correlation analysis, principal component analysis（PCA）, and salt stress threshold evaluation were conducted. 【Result】 With the increase of brackish water salinity, both FW and DW of watermelon seedlings showed a decreasing trend. FW decreased significantly by 33.38% and 38.08% compared with that of CK at salinities of 4.0 and 4.5 g/L respectively（P＜0.05, the same below）. Cl^-accumulation in the root system increased significantly by 348.43% to 492.52% at salinities of 3.0 and 4.5 g/L compared with the CK, while it peaked in stems and leaves under 4.5 g/L salinity. K⁺ accumulation in different tissues decreased with increasing salinity, with significant reductions of 52.15%, 52.16% and 54.61% in roots, stems and leaves respectively under 4.5g/L salinity compared with CK. Na⁺ accumulation in different tissues and whole plants of watermelon seedlings increased with increasing salinity of nutrient solution. K⁺/Na⁺ values in roots, stems and leaves significantly decreased by 59.68%, 61.63% and 52.70% under nutrient solution salinity of 4.5 g/L compared to CK respectively. The SPAD value of watermelon seedling leaves gradually decreased with increasing nutrient solution salinity, showing a linear correlation. SOD and CAT activities and Pro content in the leaves of watermelon seedlings showed quadratic, linearly decreasing and parabolic relationships with the salinity respectively. The MDA content in both roots and leaves of watermelon seedlings increased with increasing brackish water salinity, with significant increases of 27.30% and 25.83% in roots, and 19.07%and 12.71% in leaves, compared with CK and 3.5 g/L salinity, respectively. Correlation analysis and PCA results showed a certain correlation between FW and ion accumulation in all tissues of watermelon seedlings. The cumulative contribution rate of the first principal component（PC1） and the second principal component（PC2） reached 67.8%. Salt stress threshold analysis and evaluation showed that the salinity thresholds under mild, moderate, and severe salt stress for watermelon seedlings were 2.80-3.91 g/L, 3.91-5.32 g/L, and ＞ 5.32 g/L respectively. 【Conclusion】 Under salt stress, Na⁺ and Cl^-accumulation in watermelon seedlings reduces K⁺ uptake and destroys osmoregulatory substances and antioxidant enzyme systems, causing membrane lipid oxidation and seedling biomass decrease. Leaf Pro content, stem Cl^-accumulation, and leaf and root MDA contents can be used as comprehensive evaluation indicators for watermelon seedlings subjected to salt stress.