<p>To investigate the corrosion behavior and mechanisms of 3Cr steel under high-salinity, high-Ca²⁺, and O₂–CO₂ conditions, we simulated the service environment of a western China oilfield (total salinity 250,022&#xa0;mg/L, Ca²⁺ &gt; 15,400&#xa0;mg/L). Immersion and electrochemical tests were conducted at 120&#xa0;°C under 0.5&#xa0;MPa CO₂ with O₂ partial pressures from 0 to 0.4&#xa0;MPa. The results showed that the uniform corrosion rate increased sharply from 0.6492&#xa0;mm/a in pure CO₂ to 4.8551&#xa0;mm/a at 0.4&#xa0;MPa O₂, with localized corrosion notably aggravated. It was found that the high Cl⁻ concentration acted as a crystallization inhibitor, forcing the FeCO₃ corrosion product into an amorphous state. In pure CO₂, the corrosion film displayed a multilayered architecture: a surface crystalline CaCO₃ layer with micropores exhibiting inward/outward growth, an intermediate amorphous FeCO₃ layer, and an inner amorphous Cr-rich oxide layer. This structure was governed by the kinetic dominance of Ca²⁺, which preferentially precipitated as CaCO₃, thereby suppressing FeCO₃ growth through interfacial competition and localized acidification. With O₂ introduction, the synergistic effect of O₂, Ca²⁺, and Cl⁻ triggered the formation of a loose, porous Fe₂O₃/CaCO₃ composite scale. Notably, 3Cr steel exhibited a weak deposition behavior of CaCO₃ because of both the lower deposition tendency and adhesion strength of CaCO₃ on the amorphous Cr-rich layer. This structural deficiency led to non-uniform film coverage and generated a “small-anode/large-cathode” configuration, accelerating lateral pitting propagation. Although the dense Cr-rich layer effectively hindered Cl⁻ penetration into the substrate, it cannot prevent the expansion of acidified occluded cells. These findings elucidate the synergistic effects of O₂–CO₂–Cl⁻–Ca²⁺ on 3Cr steel corrosion and provide a theoretical basis for material selection and protection of tubing steels in oxygenated, high-salinity oil and gas wells.</p>