Abstract: In this study, the seedlings of Mesembryanthemum crystallinum L. were used as plant material. After treatment with NaCl, NaCl + CaCl₂, and NaCl + LaCl₃, the contents of Na⁺, K⁺, Ca²⁺, and Mg²⁺ in the leaves, stems, and roots were tested using ICP-OES, with the values of K⁺/Na⁺, Ca²⁺/Na⁺, and Mg²⁺/Na⁺ also calculated to explore the role of calcium in maintaining ion balance under NaCl stress. The fluxes of Na⁺ and K⁺ were also determined by the non-invasive micro-test technique (NMT). Results showed that after NaCl treatment, the content of Na⁺ in the three plant organs increased, whereas the contents of K⁺, Ca²⁺, and Mg²⁺ decreased, as did the ion content ratios. The addition of CaCl₂ reduced the content of Na⁺, but increased the contents of K⁺, Ca²⁺, and Mg²⁺ as well as the ion content ratios. Conversely, LaCl₃ treatment caused the opposite results. After treatment with NaCl for 24 h, significant effluxes of Na⁺ and K⁺ were found in the root tip, indicating that M. crystallinum L. was in a state of ionic imbalance. After the addition of CaCl₂, the efflux rate of Na⁺ increased obviously, whereas that of K⁺ was inhibited. Conversely, the addition of LaCl₃ reduced Na⁺ efflux but accelerated K⁺ efflux. The above results suggested that when M. crystallinum L. exposed to salt stress, calcium promoted Na⁺ efflux but reduced K⁺ efflux in the root tips, and maintained lower Na⁺ content in the three plant organs, suggesting that calcium plays an important role in maintaining and regulating ionic balance.