Abstract: Based on chemical thermodynamics equilibrium analysis, species distribution of As, Se and Pb in coal-fired flue gas was calculated and analyzed, and effect of S, Cl on them was studied. The results show that As exists in the form of As₂O₅, As₄O₆ and AsO in oxidizing atmosphere, Se mainly exists in the form of SeO₂, and Pb is mainly solid PbSO₄ below 1000 K and gaseous PbO above 1200 K. In reducing atmosphere, As is solid As₂S₂ at lower temperature; As₂, AsS and AsN coexist at 900-1400 K, and all of As is changed into gas AsO above 2000 K. Se exists mainly as gaseous H₂Se below 1100 K, SeS and Se₂ are generated at 1100 K, gaseous Se and a small amount of gaseous SeO at 1800 K. Pb mainly exists as PbS and gaseous Pb above 1800 K at middle and low temperature. The ratio of AsS(g), PbS(g) and SeS(g) increases in the reducing atmosphere, but the distribution of As, Se and Pb is not affected in the oxidizing atmosphere. Cl has little effect on As and Se in both oxidizing and reducing atmosphere, but has a great influence on species distribution of Pb.