The corrosion mechanism of Q235 steel by the synergistic effect of high concentration Cl− and complex scale in the preparation process of a mixed salt oil displacement method was investigated using measurements of weight loss, electrochemical impedance spectroscopy, linear polarization curve and open circuit potential. Scanning electron microscope and energy dispersive spectroscopy results indicated that the amount of Na2CO3 in the preparation process had an influence on the composition of the complex scale. This study investigated the corrosion process on Q235 steel using solutions with three different CO32−/Cl− molar ratios R: 0.11, 0.27, and 0.37. At a low concentration of CO32−, serious pitting corrosion occurred and the composition of corrosion product was diverse. When the R was 0.27, a discontinuous passive film was formed on the metal surface, and crevice corrosion took place between the adsorbed Si scale and substrate. With a further increased in CO32− concentration, the steel surface was covered with a passive film that was mainly composed of FeCO3 and Fe2O3. The initial growth process of calcium carbonate was restricted. At the R = 0.37 solution, corrosion was successfully inhibited, making this a good candidate for use in the oil recovery process.