Levulinic acid (LA) is an important platform compound that can be obtained from biomass resources. Using corncobs as the raw material that had already removed the hemicellulose, this work studied the relevant hydrolysis kinetics. The kinetic experiments were performed at various temperatures in the range of 150 °C to 180 °C and sulfuric acid concentrations between 0.2 mol/L and 0.8 mol/L in a reactor designed by the authors. The highest yield of LA was obtained at 150 °C and 0.8 mol/L H2SO4 at 58.0 mol%. A new simple kinetic model that consists of four first-order reactions was proposed; the model assumes that humins can be only converted from 5-hydroxymethylfurfural (5-HMF). A modified parameter fitting method that contained equality constraints and a weighted objective function was applied in this study. The kinetic model was in excellent agreement with the experimental data.