In view of the excellent prospects of gene therapy and the potential safety and immunogenicity issues challenged by viral vectors, it is of great significance to develop a nonviral vector with low toxicity and low cost. In this work, we report a chitosan nanoparticle (CSNP) to be used as a gene vector prepared through a facile solvent-exchange strategy. Chitosan is first dissolved in ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM Ac), and then, the solvent is exchanged with water/phosphate-buffered saline (PBS) to remove ionic liquid, forming a final CSNP dispersion after ultrasonication. The prepared CSNP shows a positive surface charge and can condense green fluorescent protein-encoding plasmid (pGFP) at weight ratios (CSNP/pGFP) of 5/1 or higher. Dynamic light scattering size and ζ-potential characterization and gel retardation results confirm the formation of CSNP/pGFP complexes. Compared with plain pGFP, efficient cellular internalization and significantly enhanced green fluorescent protein (GFP) expression are observed by using CSNP as a plasmid vector. Benefitting from the intrinsic biocompatibility, low cost, low immunogenicity, and abundant sources of chitosan, as well as the facile preparation and the efficient gene transfection capacity of CSNP, it is believed that this CSNP could be used as a nonviral gene vector with great clinical translational potentials.