The paper aims to quantify the synergistic effects of erosion-corrosion at different Cl– concentrations. The corrosion law of 20 # steel under different Cl– concentrations was obtained by means of corrosion weightlessness, morphology observation, electrochemical analysis (including Tafel curve test and impedance spectrum test) in simulated oilfield produced fluid with rotating cylindrical electrode experimental device. Based on the mass loss rates and electrochemical fitting results of pure erosion, pure corrosion and erosion-corrosion at different Cl– concentrations, the contributions of pure corrosion, pure erosion and their interactions to the total erosion-corrosion were obtained. In the simulated oilfield production fluid, when the NaCl concentration was less than 4.5%, the erosion and corrosion quality loss rate increased with the increase of Cl– concentration, and the highest rate increased to 0.44 mg/(cm2?h). Since the same potential protection was applied at each Cl– concentration, the mass loss due to erosion did not change much. It can be seen from the image that corrosion pits gradually appeared on the surface of the material with the increase of Cl– concentration. Cl– concentration has obvious promoting effect on material corrosion, and there is a critical Cl– concentration. The erosion-corrosion mass loss rate is in good consistency with the observation of morphology, and the erosion-corrosion under different Cl– concentrations is a mixed control of electrochemical corrosion-erosion, in which electrochemical corrosion dominates. As for the synergistic effect, with the increase of Cl– concentration, the corrosion promoting effect on scour gradually increases, while the erosion promoting effect on corrosion first increases and then decreases.