The dispersion behaviors of multi-walled carbon nanotubes (MWCNTs) in water, using an ionic-gemini type molecule, 4, 4'-di (n-alkyl) diphenylmethane disulfate salt (C-n-DSDM, n = 12, 14, 16) as the dispersant, were investigated. The MWCNTs reinforced magnesium (Mg) matrix composite was fabricated successfully by using the MWCNTs dispersed by DSDM. The effects of hydrophobic chain length, sonication time and pH on the dispersibility of MWCNTs were characterized by UV-vis spectrophotometer, dynamic light scattering, Raman spectroscopy, transmission electron microscopy and zeta potential measurements. Results show that the C-n-DSDM can effectively disperse MWCNTs in water through a noncovalent functionalizing mechanism, and the dispersion is enhanced with reducing in the hydrophobic chain length, i.e. the C-12-DSDM shows the best dispersibility among the three kinds of C-n-DSDM. The pH value of water for the most effective dispersion is 7, and the dispersibility is improved with the increase in sonication time. The tensile test result of the C-12-DSDM dispersed MWCNTs reinforced Mg matrix composite demonstrates that the mechanical properties especially the elongation are outstandingly enhanced, i.e. by addition of 0.1 wt% C-12-DSDM dispersed MWCNTs, the ultimate tensile strength (UTS) and elongation (E) of pure Mg are improved by about 12.5% and 50.7%, respectively. The strengthening mechanism of C-12-DSDM-MWCNTs/Mg composite was discussed in detail.