Abstract: Rust layers formed on the surface of Q235 steel subjected to a high temperature and high humidity marine atmosphere in Xisha Islands were characterized by electron probe micro analyzer,Raman spectrum and so on.The results showed that the rust layer after one-month exposure was thick and there were many cracks and holes in it.The rust layer exposed for three months was thinner than one month's.The outer rust layer of carbon steel was mainly composed of β-FeOOH,γ-FeOOH,α-FeOOH and a little Fe₃O₄ after one-month exposure.Iron oxides such as Fe₃O₄ and γ-Fe₂O₃ were found in the inner rust.The evaporation of electrolyte pricked up in the inner of a loose rust layer with exposure time.Much FeOOH which was oxidized from iron oxides was found in the inner rust after three-month exposure.Some γ-FeOOH transformed to α-FeOOH at the same time.Q235 steel continuously corroded in the Xisha atmosphere,which was caused by the recycle of oxidizing and deoxidizing of the rust.