The long-term intrusion of leachate, rich in various cations and anions, can lead to the degradation of the impermeability of landfill liners, resulting in leachate leakage that contaminates surrounding soil and water environments. In order to improve the anti-seepage performance of loess as a liner material, we conducted permeability tests on artificially prepared landfill leachate to study the impact of leachate on the permeability of loess. The study further examined the improvement effects of varying dosages of calcium lignosulfonate on the permeability of loess and its adsorption capacity for heavy metal ions. Additionally, microscopic tests including Zeta potential measurements, X-ray fluorescence spectroscopy (XRF), and scanning electron microscope (SEM) tests were conducted to elucidate the impermeability mechanism of calcium lignosulfonate-modified loess against leachate and its adsorption mechanisms for heavy metals. The results indicate that a higher compaction degree significantly reduces the permeability coefficient of loess; conversely, an increased leachate concentration elevates it. As the dosage of calcium lignosulfonate increases, the permeability coefficient of modified loess gradually decreases. Over time, the removal efficiency of Pb2+ gradually declines and stabilizes, while the modified loess demonstrates an approximately 40% improvement in Pb2+ removal efficiency. The calcium lignosulfonate-modified loess enhances Pb2+ adsorption capacity by constricting fluid flow pathways through multiple mechanisms.