Ppb amount of SO₂ in air will react with cathode material of solid oxide fuel cell, inducing decrease of the cell/stack performance as one of the most effective factors.The sulfur poisoning behavior of (La_0.6Sr_0.4) (Co_0.2Fe_0.8)O₃(LSCF) and A?site deficient (La_0.6Sr_0.4)_0.85(Co_0.2Fe_0.8)O₃(LSCF85) was investigated after exposure to 30 μg/g SO₂ at 800 ℃ for 50 h.The products of the reaction were characterized by XRD,SEM and EDX to evaluate the effect of A?site defects on the sulfur poisoning behavior of perovskite ferrite based solid oxide fuel cell cathode materials. It was found that A?site defect in LSCF85 can suppress the chemical reaction between LSCF based cathode and SO₂. It was ascribed to the low Sr reaction activity with SO₂ caused by the existence of A?site defect.