In order to determine the proper salt concentration of brackish water for irrigating cotton seedlings, this study used the pot experiment to examine the effects of brackish water under different NaCl concentrations on the physiological characteristics of cotton seedlings grown on seashore saline soil. The results showed that compared with CK, the germination rate of the cotton seeds treated by brackish water with 4 g/L NaCl decreased significantly by 26.09%, but the brackish water with 2 g/L and 3 g/L NaCl did not inhibit the germination rate of cotton seeds. Compared with CK, the fresh weight, plant height, root length and dry weight of cotton seedlings treated by brackish water with 4 g/L NaCl decreased significantly by 34.08%, 27.27%, 38.26% and 35.29%, respectively. Brackish water decreased chlorophyll contents but increased antioxidant enzyme activities, superoxide anion production rate, H₂O₂ content and MDA contents in cotton seedling leaves. The inhibition effect of brackish water with 4 g/L NaCl were the most significant, and some growth and physiological indexes of cotton seedlings treated by brackish water with 3 g/L NaCl were also decreased significantly. Nevertheless, the inhibition effects of brackish water with 2 g/L NaCl were not significant, which is the suitable concentration of brackish water for irrigating cotton seedlings in the yellow river delta.