In recent years, ionic liquids (ILs) have become a promising solvent for cellulose pretreatment in biorefinery. However, almost all the ILs that can dissolve cellulose have an unsaturated heterocyclic cationic structure, while the ILs with cations of a saturated ring can hardly dissolve cellulose. To reveal the underlying mechanism, four kinds of ILs composed of unsaturated and saturated cations (1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-l-methylpyrrolidinium, and 1-butyl-1methylpiperidinium) and the acetate anion were explored as the solvents for a cellulose bunch by molecular dynamics simulation. It was shown that the cellulose bunch only dissolved in the ILs containing cations with an unsaturated heterocyclic ring. The reason lies in two aspects. One is the structure factor: the pi electron delocalization of the unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds with cellulose. The other is the dynamic effect: the larger volume of cations with the saturated heterocyclic ring results in a slow transfer of both cations and anions, which is not beneficial to the dissolution of cellulose.