Kraft lignin has the potential to replace traditional fossil resources for the preparation of high-value chemicals because it is rich in aromatic rings and active functional groups. An effective method for the pyrolysis of kraft lignin into chemicals/fuels is microwave-assisted depolymerization. A simulation model is urgently needed to illustrate the coupling effect and mechanism of lignin conversion during the depolymerization process. In this study, COMSOL Multiphysics was used to simulate the microwave-assisted depolymerization process. The results showed that microwave power had a significant effect on the electric field and temperature distribution in the microwave cavity, while the reaction time had little effect on the electric field. The effect of the nitrogen flow rate on the electric field and temperature was negligible. The intensity of the electric field, heating rate of lignin, and final temperature of lignin depolymerization increased with increasing microwave power.