Converter steelmaking produces a large amount of high-temperature flue, which contains pollutant gases (SO2, HF, etc.) and entrains fine dusts (Fe, FeO, Fe2O3, CaO, etc.), and fluorine and sulfur behaviors that were affected by dusts in off-gas are estimated by chemical equilibrium method. The behaviors of fluorine and sulfur in off-gas are very different at reducing and oxidizing atmospheres. At a reducing atmosphere, main fluorides in off-gas are SiF4(g), CaF2(s), HF, and CaF2(g); the main sulfides are FeS2(s), FeS(s), COS(g), SO2(g), and S-2(g); and CaO can effectively defluorine at T < 1000 K. At oxidizing atmosphere, main fluorides are SiF4(g), HF, and CaF2(g); the main sulfides are Fe-2(SO4)(3)(s), CaSO4(s), SO2(g), and SO3(g); and defluorination and desulfurization by dusts are completely effectively at MR >= 1 and T < 1100 K.