With the introduction of O2 during oil and gas production, the erosion-corrosion rate of tubing steels increases; the objective of this report is to explore the reason for this. Erosion-corrosion experiments were performed in environments of CO2 and CO2-O2, respectively. Macrographs, microstructures, and the compositions of erosion-corrosion scales were investigated using a digital camera, scanning electron microscope (SEM), Kevex-SuperDry energy spectrometer (EDS) and Xray diffraction (XRD). The results show that the erosion-corrosion products are composed of large FeCO3 particles and some amorphous product in the CO2 environment, while they are made up of FeCO3, Fe2O3, Fe3O4, and bits of amorphous product in the CO2-O2 environment. The interface between erosion-corrosion scales and the substrate of 3Cr steel is smooth, and Cr enrichment obviously exists in the erosion-corrosion products in the CO2 condition. However, the erosioncorrosion scale is loose and porous with little Cr enrichment in the CO2-O2 environment, which makes the protectiveness of the erosion-corrosion scale weak, and pitting corrosion occurs. The addition of O2 may destroy the protective FeCO3 scale and Cr enrichment in the erosion-corrosion scale, which may be the main reason for the decline in the level of protectiveness of the erosioncorrosion scale, making it weak in terms of preventing the corrosive medium from diffusing to the substrate.