In order to reduce the molecular weight of konjac glucomannan (KGM), and enhance its application in the fields of food, medicine and materials, we adopt the laser combination with hydrogen peroxide to degrade the KGM solution of 0.01 g/mL. Viscosity meter and rheometer are used to determine the better degradation conditions, and then differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy are used to characterize the selected KGM degradation products, and the degradation mechanism is studied preliminarily. The results show that the viscosity of KGM decreases from 7.2 Pa·s to 3.17 Pa·s when the laser power is 10 W, the volume fraction of hydrogen peroxide is 1.5%, and the treatment time is 2 min. For the KGM obtained under the above condition, the intersection point of storage modulus and loss modulus moves backward, the intermolecular entanglement weakens and the molecular mass reduces. The temperature corresponding to thermal absorption peak of DSC increases from 301 ℃ to 326 ℃, and the thermal stability rises. There are no significant changes in the infrared spectra of KGM before and after degradation, and the characteristic functional groups remain unchanged. This study can provide a new idea for the effective degradation of KGM.