To clarify the mechanism of soot formation is very important from the viewpoint of reduction of environmental load and energy saving. Despite the large number of experimental and theoretical studies made on soot production, the mechanism of soot formation remains unclear. In this article, benzene pyrolysis was carried out in an alumina tube heated by plasma, and a numerical simulation was performed with a kinetic model that takes into account the reactions of benzene pyrolysis, nucleation formation, surface growth and coagulation and was constructed by adopting the Discrete-Sectional Method. The effects of residence time and initial benzene concentration on soot formation rate were examined. The results show that soot particles have a larger diameter and a narrower size distribution with the increase in residence time and reactive gas concentration.