The effect of pretreatment with acidic bronsted ionic liquid 1-sulfonic acid butyl-3-methylimidazolium trifluoromethanesulfonate ([B(SO3H)mim]OTf) on Shengli lignite (SL) pyrolysis behavior and its pyrolysis products distribution were investigated in this paper. In order to study the mechanism of [B(SO3H)mim]OTf pretreatment on the dissolution of SL, [B(SO3H)mim]OTf treated-SL was also analyzed by the Fourier Transform Infrared Spectroscopy (FTIR), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) and TG-DTG. It is found that the pretreatment temperature and the ratio of [B(SO3H)mim]OTf to SL have a significant impact on the pyrolysis of SL. The pretreatment of SL with [B(SO3H)mim]OTf can not only result in the increase of the yield of total tar and light oil fraction but also improve the quality of tar. After the pretreatment at 200 degrees C and [B(SO3H)mim]OTf/SL at 1, the tar yield, light fraction content, arenes, and phenolics content in tar is 1.6, 2.4, 6.3 and 3.2 times higher than that from raw SL pyrolysis, respectively. Pretreatment with [B(SO3H)mim]OTf leads to the fracture of the coal network, resulting in the increase of the content of C=O and C-O-C bonds and the disruption of hydrogen bonds.