The effects of oxidant dosage, oxidation temperature and time on the degradation of soda lignin by hydrogen peroxide with and without the presence of microwave irradiation were investigated. It is found that the oxidative degradation of lignin includes the cleavage of ether bond in beta-O-4 structure, the partial destruction of aromatic ring, and the re-condensation of the degraded lignin. Compared to the conventionally heated oxidation of lignin, the microwave irradiation efficiently facilitates the degradation of the lignin with high molecular weight and the re-condensation of that with low molecular weight at a low oxidant dosage, low oxidation temperature, or a short oxidation time, which leads to the formation of the degraded lignin with narrower molecular weight distribution and lower molecular weight. Additionally, the lignin degraded in the presence of microwave irradiation has the characteristics of higher content of phenolic hydroxyl group, lower content of methoxyl group, and lower degree of condensation, which enhances the reactivity of lignin. Therefore, the oxidative degradation of lignin assisted by microwave irradiation may be a new pretreatment approach for efficiently utilizing the soda lignin.