Highly resilient lignin-containing polyurethane (PU) foam was synthesized via using alkali lignin as a partial substitute for the petrochemical polyols. The flexible polyethylene glycol chains were first grafted onto alkali lignin, converting the phenolic hydroxyls on lignin to aliphatic hydroxyls and enhancing the reaction activity of lignin. The influence of the modified-lignin on the structure, mechanical properties and thermal stability of PU foams were analyzed. The grafted polyethylene glycol chains provided flexibility to the lignin-containing PU foams. All lignin-containing PU foams exhibited large elastic recovery higher than 93%. The compression strength of the PU foams increased with the lignin content while maintaining excellent resilient performance. This work gives a new approach for the application of biopolymer lignin in flexible and highly resilient polyurethane foams.