Abstract: Through simulated polluted atmospheric corrosion system, the effect of relative humidity(RH) on the corrosion morphology and the growth process of corrosion products of 20 low carbon steel in atmosphere containing 5×10-6 volume fraction SO2 was studied by means of optical microscopy, SEM, XRD and XPS. The results show that the corrosion products were mainly composed of α-FeOOH, γ-FeOOH, FeSO4, Fe3O4, γ-Fe2O3 under the enviroment of different relative humidities, and the concentration of FeOOH increased with increase of RH, and the corrosion rate increased, too. When the RH were 65% and 75%, the corrosion curves was divided into two stages which respectively followed by exponential increase and the linear growth. When the RH were 85% and 95%, the corrosion curves followed by linear growth. At the enviroment of RH of 65%, the corrosion products grew in the form of corrosion-ring. With increase of RH, the filiform corrosion products were found around the corrosion-ring. When the RH was 95%, it was filiform corrosion products with cellular corrosion products, and obvious pits were observed under the cellular corrosion products.