Abstract: Through analyzing the composition and structure of both oligomers and polymers of styrene by GCMS and FT-ICR MS methods, a mechanism of styrene polymerization occurred in the extraction distillation process was proposed. It is concluded that the styrene polymerization is triggered along the radical initiation and thermal initiation with the latter one as a dominate way. It is found that 1,3,5-triphenylcyclohexane is participated in the chain initiation reaction and that the ethylbenzene in extraction feedstock is involved in the chain transfer reaction of styrene polymerization, resulting in more ethyl phenyl free radicals in the system.