Ionic liquids have unique advantages as catalysts in petroleum desulfurization，especially for thiophene sulfides. In this paper，tetrahydrothiophene （THT） was selected as a model compound and hydrogen peroxide as the oxide. Using density functional theory，ionic liquid was selected as catalyst，and the catalytic mechanism in the oxidation of tetrahydrothiophene to sulfone was studied. Cationic chain length was changed，and the influence of different side chains on catalytic effect was explored. The selective complexation mode of ionic liquids with reactants and products showed that ionic liquids had stronger interaction with the oxidation product sulfone. The longer the cationic side chain，the more stable the complex structure. The charge population and the properties of Frontier Molecular Orbital of the complex showed S atom on THT had charge enrichment，while the electrons transferred from THT to the ionic liquid during the complexation process. The influence of ionic liquid on the bond energy，and charge population of the reactants was explored at molecular level. The energy barrier of the reaction process proved ionic liquid reduced the reaction energy barrier by more than 55%，and the longer cationic side chain，the more significant the effect.