Abstract: Salinization and alkalization of soil is one of the ecological factors seriously limiting agricultural production. This study investigated the adaptive mechanisms of salt-tolerant plants to help make full use of the abundant halophyte resources to improve the utilization of vast areas of saline and alkaline soil. Seedlings of Melilotus officinalis were treated with NaHCO₃ solution at three concentrations, 0 (control), 150 and 200 mmol/L. The anatomical structure of three organs (roots, stems and leaflets) and ultrastructure of two organelles (chloroplast and mitochondria) were observed using paraffin sections and ultrathin sections. The results showed that NaHCO₃ treatments significantly affected the vascular system of M. officinalis. With NaHCO₃ treatments, vascular tissue throughout the roots, stems and leaflets of seedlings were smaller. In particular, the diameter of xylem vessels was drastically reduced which greatly limited the transportation and transverse diffusion of water. The cross section of stems was changed from near quadrangle to irregular shape, which increased the surface area of the stems and furthered opportunity for photosynthesis. Epidermis cell wall cutinization occurred and the diameter of xylem vessels in vascular bundles greatly decreased which reduced water loss through stomata and non-stomata pathways. Chloroplast in stems and leaflets and mitochondria in root tissue were only slightly damaged. Thylakoid in chloroplasts and ridges in mitochondria still had an intact membrane system and a sharp-edged laminated structure. All changes indicated that structural integrity of organelles was protected by active structural morphological changes in M. officinalis tissue under saline and alkaline stress.