1-alkyl-3-ethylimidazole bromide ([REhIM]Br, R=C₁₀H₂₁, C₁₂H₂₅ and C₁₄H₂₉) ionic liquids (ILs) was synthesized. The molecular structure of[REhIM]Br was optimized by density functional theory (DFT) at B3LYP/6-31 level, and the most stable configuration was obtained. The partial bond length bond angle in the molecule was obtained by theoretical calculation, and the charge distribution was analyzed. The interaction energies between anions and cations and[REhIM]Br were calculated by calculating the single point energies. Finally, the LUMO energy, HOMO energy and energy gap (ΔE_LUMO-HOMO) data in the electronic structure were obtained. The results showed that when the anion remaines unchanged, with the increase of alkyl chain of[REhIM]Br ionic liquid, the bond length, bond angle and charge distribution are independent of the length change of alkyl chain. The energy-related data such as interaction energy of ions, LUMO energy, HOMO energy and energy gap (ΔE_LUMO-HOMO) are closely related to the number of C atoms on the alkyl chain. With the increase of alkyl chain, the interaction energy of ions and energy gap become smaller, and the structure becomes unstable.