A novel hemicellulose and lignin-first process for corn stover valorization catalyzed by Al2(SO4)3 in GBL/H2O co-solvent was developed in this study. In 25% GBL/H2O at 160 °C, Al2(SO4)3 assisted H2O in breaking down intermolecular linkages of corn stover. The hydrolysis of Al2(SO4)3 could produce H+, [Al(OH)2(H2O)x]+ and SO42−. With the solvation of GBL, the H+ and [Al(OH)2(H2O)x]+ from Al2(SO4)3 hydrolysis increased the co-conversion and dissolution of hemicellulose (96.9 wt%) and lignin (68.0 wt%), while the AlHSO4(OH)2H2O species formed by combining with [Al(OH)2(H2O)x]+ and SO42− was found to inhibit cellulose conversion (88.7 wt% kept). Expecting for H+ from Al2(SO4)3 hydrolysis, the formed AlHSO4(OH)2H2O was found to be the catalytic active species for the cleavage of glycosidic bonds in hemicellulose to produce xylose, and also acted as a “stabilizer” to prevent the further degradation of xylose to improve its yield (87.6%, based on the weight of hemicellulose). Al2(SO4)3 selectively promoted lignin depolymerization to lower Mw oligomers, as well as monophenols (11.20%) with high selectivity of 70.0% to VG and VP via Cα-OH dehydration and -Cγ(=O)-OH decarboxylation reaction. The obtained cellulose-rich residues show great potential for further use within many industrial processes. The developed process is recyclable, giving important insights to design new approaches for corn stover valorization.