Four ionic liquids, 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), 1-hexyl-3-methylimidazolium acetate ([HMIM][OAc]), and 1-octyl-3-methylimidazolium acetate ([OMIM]-[OAc]), were used to swell Hefeng sub-bituminous coal. The effect of swelling on the molecular structure and pyrolysis behavior of the coal was investigated by swelling degree Q SEM, FTIR, and TG-DTG characterizations. Results showed that swelling degree by different ionic liquid was in the range of 2.0-2.4, with a higher degree of 2.4 for the coal sample swelled by [OMIM][OAc]. The swelling process could destroy the surface morphology and pore structure of the coal and increase and loosen the sample's pore channel. The results of the FTIR analysis revealed that swelling can destroy hydrogen bonds in coal, reduce the relative content of oxygen-containing functional groups, and might increase the relative amount of low-molecular-weight organics. TG-DTG profiles proposed that swelling with ionic liquid could promote weight loss and pyrolysis conversion of the coal and significantly changed its pyrolysis behavior at the temperature of 180-350 degrees C. The Coats-Redfern results presented that swelling increased the pyrolysis activation energy of the coal at 180-350 degrees C, while the energy was decreased at 350-550 degrees C.