Uniform chitosan composite nanofiber mats (CS/PAAS) containing 4.0 wt% polyacrylic acid sodium loading have been prepared by electrospinning, followed by annealing at elevated temperature to improve solvent resistance and mechanical strength. The CS/PAAS nanofiber mats have been characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and tensile strength analysis. The prepared nanofiber mats have been shown to be much better than the pristine chitosan powder to adsorb the chromium (VI) ion. SEM-Energy dispersive X-ray spectroscopic (SEM-EDS) and positron annihilation lifetime spectroscopic (PALS) analysis show that Cr(VI) ion can freely permeate into the composite nanofiber and coordinate with the internal chitosan chain molecules. The adsorption abilities of these cross-linked CS/PAAS nanofiber mats are dependent on the chitosan contents as well as N-atom basicity of the chitosan chain. The adsorption capacity toward Cr(VI) ion was improved to 78.92 mg/g after modification of the chelating ligands for the cross-linked CS/PAAS composite nanofibers.