Lignin has received increasing attention as a potential starting material for providing valuable low-molecular-weight aromatic compounds. Herein, a strategy for lignin degradation and low-molecular-weight compound hierarchical separation is presented. Ionic liquid and compressed CO2 were used in the two-step protocol. 1-Butyl-3-methylimidazolium acetate was used to induce the cleavage of C-O and C-C linkage as well as dehydration reaction of lignin at high temperatures, especially combined with sonication assistance. The low-molecular-weight aromatic compounds from lignin can be followed to separate efficiently and hierarchically by adjusting the pressure of compressed CO2. The size distribution and chemical structure of lignin samples were investigated to explain the mechanism, indicating that our method is easy and efficient.