Thiophene sulfur is the most stable organic sulfur species in petroleum coke, among which benzothiophene accounts for a significant portion. Removal of benzothiophene will help to gain ultralow desulfurization. In this work, a density function theory (DFT) method was adopted to investigate benzothiophene pyrolysis mechanism. It was found that the most possible pyrolysis reaction of benzothiophene is triggered by alpha-H migration to beta-position. The dominating products are S radical and ethenethione, which could explain benzothiophene pyrolysis experiments well. Converting thiophene fused on aromatic to a thiol group could help to promote desulfurization. As a contrast, the thiophene pyrolysis reaction was also calculated at the same level. The initial pyrolysis temperature of benzothiophene and thiophene may be close, but the pyrolysis rate of thiophene is higher than that of benzothiophene. The implication of the benzothiophene pyrolysis mechanism may be beneficial for the development of new desulfurization technology.