Abstract: 【Objective】 The study aimed to explore the effects of salt stress on root configuration，vitality and ion distribution of grafted watermelon seedlings and to comprehensively evaluate each index，providing theoretical basis for determining the salt stress threshold of watermelon and the safe utilization of brackish water. 【Method】 Pot experiment was conducted，and grafted seedlings of Jincheng No. 5 watermelon were used as experimental materials. Hoagland nutrient solution was used as the basis，with a complete nutrient solution（mineralization was 2.5 g/L）as the control（CK）. Brackish water solutions with mineralizations of 3.0，3.5，4.0 and 4.5 g/L were prepared by adding different concentrations of NaCl，NaHCO₃，and CaCl₂. The effects of different mineralizations on root configuration，vitality，malondialdehyde （MDA）content and salt ion distribution in watermelon roots were studied. Correlation analysis and principal component analysis（PCA）were used to comprehensively evaluate the root salt stress threshold. 【Result】 Compared with CK，the relative dry weight of watermelon root significantly decreased by 25.36% and 48.62% at mineralizations of 4.0 g/L and 4.5 g/L respectively（P<0.05，the same below），while the relative fresh weight significantly decreased by 34.81% at a mineralization of 4.5 g/L. Compared with CK，the root length，root surface area，root volume，root diameter，number of root tips，and number of root forks in seedlings were significantly reduced by 23.24%，6.35%，42.82%，17.37%，16.89% and 13.04% at a minera-lization level of 4.0 g/L，and significantly reduced by 37.58%，18.77%，54.34%，31.66%，17.30% and 18.45% at a mine-ralization level of 4.5 g/L respectively. The MDA content in roots increased greatly at mineralization ≥4.0 g/L，with an increase of 19.90%-23.74%. The root vitality gradually decreased with the increase of mineralization，with a reduction of 43.46%-53.76% at 4.0 g/L to 4.5 g/L. Meanwhile，watermelon seedlings accumulated large amount of Na⁺ and Cl^- in the roots，stems and leaves，with excess Na⁺ stored in the stems. The selective transport ratio of Cl^- and Na⁺（S^Cl，Na）from roots to leaves reached its maximum at a mineralization level of 3.0 g/L，significantly increasing by 33.88% compared to CK. Correlation analysis and PCA results showed that 4 indicators—root relative dry weight，root length，stem Cl^- content and leaf Cl^- content—had a cumulative contribution rate of 83.2% in the principal component analysis. Root length and root relative dry weight were positively correlated with root relative fresh weight，while stem Clcontent and leaf Cl^- content were negatively correlated with root relative fresh weight. The results of the analysis and evaluation of salt stress thresholds showed that the mineralization threshold for mild salt stress was 2.58-4.20 g/L，that for moderate salt stress was 4.20-5.35 g/L，and that for severe salt stress was greater than 5.35 g/L. 【Conclusion】 Under salt stress，the root system of watermelon seedlings alters its configuration by reducing root length，surface area，volume，diameter，number of root tips and number of root forks. The absorption of large amounts of Na⁺ and Cl^- by the root system leads to a decrease in root vitality. Root relative fresh weight，root relative dry weight，root length，stem Cl^- content，leaf Cl^- content，root Na⁺ content and number of root forks can be used as indicators for determining salt stress thresholds.