The full-system cycle corrosion behavior of E36 grade low-alloy steel was studied by a homemade device to simulate the cargo oil tank upper deck corrosion environment which was established corresponding to the international maritime organization standard. The corrosion rate and reduction of thickness of the steel were measured respectively, and an extrapolation of the thickness reduction for 25 years corrosion was calculated by fitting formula and curve. Surface morphology of steel tested for different cycles before and after the removal of corrosion product films was observed by scanning electron microscope (SEM). The distribution of element and phase constituent of corrosion product film were analyzed by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) respectively. The results showed that the extrapolation of thickness reduction for 25 years corrosion was 2.21 mm. In the early stage of the corrosion, acid condensing droplets were formed on the surface of corrosion product film due to the existence of sour gas and the alternating temperature and humidity, and then the corrosion scale was gradually blistered. In the later stage of the corrosion, the size of the blisters grew up, and then most of which cracked and falled off. At last, the outer corrosion scale was completely detached, and the inner corrosion product scale was exposed to the corrosive environment. The rather loose outer corrosion scale of the steel formed in wet O₂-CO_2-SO₂-H₂S gas environment was mainly consisted of α-FeOOH, γ-FeOOH, S, FeS₂, Fe_1-xS and FeS. However the rather compact inner corrosion product scale was consisted mainly of α-FeOOH.