Abstract: To address the dual challenges of inefficient utilization of lignite resources and cadmium-contaminated soil remediation in China, this study utilized Inner Mongolia Shengli No.5 lignite as raw material and laboratory-preserved microbial strains to prepare a lignite humic acid composite microbial agent. Industrial analysis, elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and specific surface area analysis were employed to investigate the adsorption mechanism of Cd²⁺ by lignite humic acid-microbial complexes. The results demonstrate that Penicillium janthinellum H3 significantly enhanced the mass fraction of humic acid in lignite to 18.80% (41.1% higher than raw coal) and increased oxygen-containing functional groups such as carboxyl and hydroxyl groups. The composite microbial agent exhibited an expanded specific surface area of 6.90 m²/g (versus 2.50 m²/g for raw coal), with pore volume and average pore size increased by 2.88-fold and 4.4%, respectively. After Cd²⁺ adsorption, the increased proportions of hydroxyl and ether-bonded oxygen (—OH—O) groups in the composite agent indicate that complexation and electrostatic interactions constitute the primary adsorption mechanisms. XRD analysis revealed no crystalline precipitates formed during adsorption, further corroborating that Cd²⁺ removal predominantly occurs through surface interactions rather than precipitation. Microcrystalline structure analysis suggests potential Cd²⁺ intercalation within the interlayer spaces of aromatic lamellae, enhancing structural stability. Synchronous microbial conversion for humic acid release and Cd²⁺ adsorption enhancement is achieved, revealing that Cd²⁺ immobilization is significantly strengthened by the synergistic effects of surface functional groups and porous architecture in the composite microbial agent. The research provides a “waste-to-cure” strategy for Cd-contaminated soil remediation, offering both environmental benefits and resource utilization value.