Oil tank is easily corroded by internal and external media in service. Corrosion types of gas spaces in oil tank were summarized, corrosion process was analyzed also. Through the research of corrosion mechanism of gas space of oil tank, author put forward that H2S corrosion was a dominant factor for gas space corrosion. H2S corrosion mechanism model at low temperature was obtained also. Metallic samples test showed that corrosion rate was decreased while the samples were placed in simulating atmosphere spaces of oil tank. Corrosion product iron sulfide has loose, porous, layered, and cracked structure. When ingredient of atmosphere varied with time，iron scale was produced, mixed with iron sulfide. Electrochemical test showed that the increase of acidity accelerated the cathode process and accelerated the corrosion rate of Q235-B steel.