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Soil salinization is a major environmental stressor that reduces the growth and yield of crops. Maintaining the balance of ions under salinity is vital for plant salt tolerance; however, little is known about the correlation between the salt tolerance of crops and the ion contents of their roots and shoots. Here, we investigated the poorly understood salt-tolerance mechanisms, particularly regarding ion contents (particularly Na⁺), in <i>Brassica napus</i> subsp. <i>napus</i> L., an agriculturally important species. Twenty <i>B. napus</i> inbred lines were randomly chosen from five salt-tolerance categories and treated with increasing concentrations of NaCl (0–200 mmol) for this work. We found that the root Na⁺ content is the most correlated limiting factor for the salt tolerance of <i>B. napus</i>; the higher the salt tolerance, the lower the root Na⁺ content. Correspondingly, the Ca²⁺/Na⁺ and K⁺/Na⁺ ratios of the roots were highly correlated with <i>B. napus</i> salt tolerance, indicating that the selective absorption ability of these ions by the roots and their translocation to the shoots play a pivotal role in this trait. These data provide a foundation for the further study of the molecular mechanisms underlying salt tolerance and for breeding salt-tolerant <i>B. napus</i> cultivars.