This study demonstrated the successful preparation of high lignin-based phenolic foam (LPF). Alkali lignin (AL) was first activated via depolymerization by a solid acid-catalyzed phenolation process. Characterization of the phenolated AL (PAL) indicated that the phenolation significantly reduced the molecular weight of AL and generated more phenolic hydroxyl groups in PAL. Then, phenolic foams (PFs) were prepared using PAL and AL to substitute phenol. The results indicated that the PAL-based PF (PPF) and the AL-based foam (APF) had structures very similar to the pure phenol-based foam (PF). The LPF foams (PPF and APF) showed lower volumetric water absorption, lower slag rate, and better thermal stability than the PF foam. PPF had higher compressive strength and a more uniform closed cell structure than the APF at the same lignin substitution rate, suggesting that the phenolation activation improved the reactivity and performance of lignin in the preparation of LPF.