The dominant species of the halophytes Leymus mollis(dune grass) and Calystegia soldanella (beach bindweed) grown in the coastal dunes of the coast of Yantai, China were chosen as materials for this study. Soil and plant materials were sampled from the high tide line, and 10, 20, 30, 40, and 50 metres off the high tide line of the coast. Na⁺ concentrations, activities of the antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and malondialdehyde (MDA), proline, soluble protein, and sugar contents in the roots and leaves of L. mollis and C. soldanella were determined to understand the role that Na⁺ accumulation in the roots and leaves played in the physiological adaption to saline environments. Na⁺ concentration in the soil was highest at high tide line, and declined with distance, with the sharpest decrease between the high tide line and 20 m further inland. L. mollis and C. soldanella both grew between 10 m and 20 m off the high tide line and further inland. At distances close to the high tide line, the roots and leaves of L. mollis had higher Na⁺ concentrations than did those of C. soldanella;both species had low contents of MDA and water and lower activities of antioxidant enzymes but higher concentrations of the osmoregulatory substances in leaves and roots.Farther from the high tide line, Na⁺ concentration also increased in the roots of C. soldanella; at 50 m, Na⁺ was 637% higher in roots and 319% higher in leaves than in those tissues in L. mollis. Far away from the high tide line, the contents of water and MDA increased in the leaves and roots of both plants; the activities of POD and SOD increased in the roots; and the contents of soluble sugar and proline decreased in the roots of both plants. However, at different distances from high tide, L. mollis had lower average water content, higher contents of MDA, proline, and soluble sugar, and higher POD, CAT, and SOD activity. At the saline seashore soil, L. mollis maintained free radical metabolic balance and water balance by controlling Na⁺ uptake and increasing the activity of antioxidant enzymes and the content of organic osmoregulatory substances. However, C. soldanella is a salt-secreting halophyte with higher Na⁺ and average water content in its leaves and roots, lower MDA content, activities of POD and CAT, and lower contents of proline and soluble sugar on different locations away from high tide, which indicated that the Na⁺ accumulated in the leaves and roots of C. soldanella played a very importance role in maintaining ion balance. Na⁺ accumulated in the roots and leaves of L. mollis and C. soldanella can induce the accumulation of oxygen free radicals to activate the antioxidant enzyme system which in turn would inhibit lipid peroxidation to maintain the balance of oxygen free radical metabolism. Na⁺ accumulated in the roots and leaves of two plants not only can be an inorganic osmotic adjustment substance to maintain cellular water balance, but plants also can increase organic osmotic adjustment substances such as proline and soluble sugars to maintain water balance. Results imply that Na⁺ in the leaves and roots of both plants plays a very important role in physiological regulation of the balance of oxygen free radical metabolism and water balance, allowing these two plants to survive seashore saline conditions.