Chitosan dissolution system with [Emim]Ac (1-​ethyl-​3-​methylimidazolium acetate) and urea was designed for homogeneous chemical modification of chitosan. Physicochemical properties of [Emim]Ac and [Emim]Ac-urea, such as density, solvatochromic parameters etc, were thoroughly investigated. NMR and FTIR spectra of [Emim]Ac-Urea solution were tested for determination of interaction between [Emim]Ac and urea. Obvious chemical shift change (Δd) of typical atoms (H2, H11, C10, and C12) in [Emim]Ac-urea indicated that strong hydrogen bonds were formed between [Emim]Ac and urea. Chitosan dissolution performance in [Emim]Ac-Urea solution was studied. Density functional theory (DFT) simulations were carried out to discuss the interactions between [Emim]Ac, urea, and chitosan. Four kinds hydrogen bonds (CH/O, CH/N, NH/O, NH/N), and eight hydrogen bonds, were formed in chitobiose-[Emim]Ac-urea. In particular, anion acetate of [Emim]Ac, hydrogen atoms and nitrogen of urea formed strong hydrogen bonds with oxygen atoms, amide, and hydroxyl in chitobiose. The information obtained would provide a guide for the development of novel solvent systems for chitosan dissolution and homogeneous chemical modification.