A carbon-based solid acid catalyst (CHACS) derived corncob residual was prepared by incomplete hydrothermal carbonization followed by activation with phosphoric acid impregnation and sulfuric acid sulfonation. The structure of the solid acid catalyst was characterized using Fourier transform infrared spectra (FTIR), thermogravimetric analyzer (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), specific surface area (SSA), and elemental analysis. The -SO3H, -COOH, and phenolic -OH functional groups were successfully introduced. Phosphoric acid activation facilitated formation of the porous structure in the solid acid catalyst. The specific surface area and acid density were 1569 m2/g and 1.030 mmol/g, respectively. The CHACS exhibited better catalytic activity for hydrolysis conversion of corn stalk in ionic liquid with water as solvent. A total reducing sugars (TRS) yield of 68.3% was obtained in water and a TRS yield of 52.5% was obtained in an ionic liquid of [BMIM][Cl] at 140 °C for 120 min. The CHACS expressed good catalytic activity in each of 4 separate instances of reuse.