The corrosion behavior of 20# steel was studied under CO₂/H₂O two phase plug flow condition using self-designed experiment device. The corrosion rate, corrosion appearance, the composition of corrosion products and the structure characteristics of film were investigated through weight loss method, scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and X-Ray diffraction (XRD), respectively. The results show that the trend of corrosion rate first slightly decreases followed by a rapid increase, and then dramatically decreases with the increasing of corrosion time. The maximum (2.074 6 mm/a) and minimum (1.898 8 mm/a) value reached at 4 h and 8 h, respectively. The corrosion characteristics of corrosion products on the bottom pipe wall by the beginning of the single layer of loose flocculent and tine needle product change into double films of corrosion product gradually. The outer layer with micro-crack is loose, but the inner is relatively dense and its density is increasing with time. The EDS analysis results confirm that the inner dense layer belongs to rich iron product because the Fe content is higher than that of outer layer, and the content of C/O elements in outer loose layer is relatively high. At the same time period the corrosion products at the top wall of pipe are a zonal distribution along the vertical direction of flow, and the bulky loose particles at initial stage gradually transform into crystal grain with regular arrangement. Corrosion products mainly consist of Fe, C, O and the composition mainly includes Fe₃C, FeCO₃, Fe₃O₄ and FeOOH.