The influence of sulfur dioxide addition on dioxins generation and decomposition behaviors has been evaluated by thermodynamic equilibrium calculations. The calculation conditions in this work were as follows: (1) the C-H-O-Cl quaternary system has the ratio of [C : H : O : Cl]=9 : 6 : 4 : 1, i.e. this system corresponds to the most typical generation region of dioxins, (2) total amount of respective elements (N_C+N_H+N_O+N_Cl) is 1 mol, (3) sulfur dioxide is then added to this C-H-O-Cl quaternary system and (4) total pressure and temperature range were 1 atm and from 773 to 1773 K. The influence of sulfur dioxide addition is shown to change the oxygen potential, on which the dioxins generation and decomposition is independent, i.e. higher temperature and higher oxygen potential lead to dioxins decomposition. The essential effect of sulfur element is shown to reduce hydrogen chloride potential, leading less dioxins chlorination reaction.