Etherification mechanism of 4,5-dihydroxy-1,3-bis (hydroxymethyl) imidazolidin-2-one (DMDHEU) with the primary alcohols and the –OH hydroxyl groups of cellulose chain (n=1-2) in acidic condition were investigated by using density functional theory (DFT) method and a two-layer ONIOM approach. Geometry and energy of reactants, products, intermediate complexes, carbocation intermediate, and transition states were optimized at B3LYP/6-311g(d,p) level and ONIOM (B3LYP/6-311g(d,p):PM3MM) level. Computational results indicate that the etherification adheres to unimolecular nucleophilic substitution (SN1) mechanism; the reactant and product can form the activated complexes with H+ ions in which H+ ions are occupied by the O-atom of C=O group in the reactant complex and the product complex. Potential energy surface (PES) of the reaction has the triple-well shape. Effect of substituent R in primary alcohol R-CH2OH (R = H, CH3, CH2CH3, CH2OCH3, CH2F) and cellulose chain on the reactivity is significant. The energy barrier of H+ ions releasing step is much higher than those of the activation steps. The calculational data is in the good agreement with the experimental data in the literature.