In this study, a carbon shell was coated on Fe₃O₄ nanoparticles using a hydrothermal method followed by modification of carboxyl end groups on the Fe₃O₄@C to form Fe₃O₄@C-COOH for creating an immobilisation agent for remediating lead-contaminated soils. The surface of an Fe₃O₄@C nanoparticle was successfully covered with carboxyl end groups. The Fe₃O₄ core possessed the superparamagnetism property; the carbon shell protected the core from being oxidised or dissolved in acid solution, and provided good modifiability. Due to the strong interaction between lead and carboxyl end groups, this synthesised remediation agent exhibited high adsorption capacity. The Fe₃O₄@C-COOH nanoparticles principally promoted the transformation of lead (Pb) from a reducible to residual state, while having no obvious effect on the oxidation state of the lead. The amount of Fe₃O₄@C-COOH and the composition of soil organic matter had a higher influence on Pb distribution than soil pH, water content, or conductivity. Under optimal immobilisation conditions, the fractionation of the Pb acid-soluble, reducible, oxidative, and residual states in the contaminated soil changed significantly. The leaching and migration of Pb were significantly reduced, thus achieving remediation of lead-contaminated soils by immobilisation. Thus, remediation of lead-contaminated soils via Fe₃O₄@C-COOH immobilisation is a potentially practical and technologically feasible method.