Objective: As offshore oil and gas exploration and development progressively advance into deepwater areas, wellbore integrity management faces severe technical challenges, and existing chemical plugging systems are unable to fully satisfy the performance requirements. This study aims to develop a novel plugging agent that integrates self-settling characteristics, high strength, and long-term stability, thereby providing a reliable solution for wellbore integrity restoration in offshore oil and gas fields. Methods: Lignin was first modified via hydroxymethylation with formaldehyde under alkaline conditions to introduce reactive hydroxymethyl groups, and then copolymerized with phenol and formaldehyde to yield the lignin-modified phenolic resin plugging agent precursor. The structure and thermal stability of the plugging agent were characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Injectivity and self-settling performance were evaluated using a rotational rheometer and a density meter. Compressive strength of the cured samples was measured with a universal testing machine. Gas sealing performance was assessed using a full-scale annular simulation device to verify the plugging effectiveness under conditions approximating actual working environments. Results: FT-IR analysis confirmed the successful hydroxymethylation modification of lignin. TGA demonstrated that the plugging agent remained structurally stable below 300℃, with a total mass loss of only 3.6%. Under the optimal formulation (formaldehyde 30 mL, phenol 40 mL, lignin 10 g), the agent exhibited a density of 1.18 g/cm3, enabling self-settling in the annulus, and a viscosity of 103 mPa·s at a shear rate of 170 s?1, displaying favorable pumpability and shear-thinning behavior. The cured samples achieved an initial compressive strength of 12.5 MPa, representing a 30.2% improvement over the unmodified phenolic resin (9.6 MPa). After 150 days of aging at 80℃, the compressive strength was maintained at 12.48 MPa, with a strength retention rate exceeding 99%. In the full-scale annular simulation sealing test, the plugging agent sustained a gas pressure of 6.89 MPa for 2 minutes without leakage in a narrow 5 cm annulus, and the cured body exhibited excellent bonding with the casing wall. Conclusion: This study successfully developed a novel lignin-modified phenolic resin plugging agent. The agent integrates self-settling injectivity, high strength, and long-term durability, and its reliable sealing capability under narrow-annulus and high gas pressure conditions was verified by full-scale experiments. The grafting of lignin via hydroxymethylation, which participates in crosslinking and curing, enhances crosslinking density and interfacial adhesion, constituting the core mechanism underlying the improved performance. This plugging agent can provide effective technical support for wellbore integrity management in offshore oil and gas fields.