The process of delignification during the pretreatment of corn stover in supercritical CO2 with ethanol-water as co-solvent was investigated. After pretreatment, many lignin droplets were found deposited on the fiber surface which hinder cellulose digestibility. These lignin droplets were removed by ethanol-water and after washing the optimal glucose yield increased significantly. Lignin degradation reactions competed with condensation reactions during pretreatment. The cleavage of ether bonds and the high solubility of lignin fragments in ethanol-water co-solvent were the key factors for lignin removal and degradation behavior during pretreatment.