In response to the corrosion protection requirements for Q235 carbon steel in acidic environments, this study systematically evaluated the anti-corrosion performance of Eucalyptus citriodora leaf extract (ECLE) on Q235 carbon steel in 0.5 mol/L H2SO4 solution for the first time. The primarily water-soluble constituents were extracted using an eco-friendly ethanol-based method. Subsequently, electrochemical experiments demonstrated that ECLE achieved a corrosion inhibition efficiency of 91.2% at 298 K. Meanwhile, surface characterizations further confirmed that the addition of ECLE significantly reduced the corrosion extent of Q235 steel. Furthermore, quantum calculations further demonstrated that the synergistic adsorption mechanism of ECLE on the carbon steel surface involved the coordination of deprotonated carboxyl groups (R-COO-) with iron ions, the mixed adsorption of hydrogen-bonded interfacial structure, and the physical adsorption of alkyl chains. This work demonstrates an efficient, cost-effective, and sustainable approach for industrial acid cleaning processes, contributing to green corrosion protection technologies.