Radiation-induced pulmonary fibrosis (RIPF) is a severe complication in patients treated with thoracic irradiation. Until now, there are no effective therapeutic drugs for RIPF. In the present study, we attempted to evaluate the effect of Magnesium isoglycyrrhizinate (MgIG) on RIPF, and to further explore the underlying mechanisms. We found that MgIG treatment markedly improved radiation-induced lung pathological changes, reduced collagen deposition, and decreased the transforming growth factor beta1 (TGF-β1) elevation induced by irradiation. In addition, MgIG treatment significantly relieved oxidative damage of pulmonary fibrosis in mice characterized by increased antioxidant factors expression and reduced oxidative factors expression. And, MgIG treatment also significantly reduced the production of intracellular reactive oxygen species (ROS) in vitro. Interestingly, administration of MgIG achieved lower expression levels of Nox4, and phosphorylation of p38MAPK and Akt in vivo and in vitro. Furthermore, treatment with MgIG notably reduced the expression levels of myofibroblast markers, Nox4, and phosphorylation of p38MAPK and Akt both in vivo and in vitro. More importantly, the inhibitory effects of MgIG on fibroblast differentiation were enhanced when the p38MAPK/Akt/Nox4 pathway was inhibited using their respective antagonists or Nox4 siRNA in vitro. Taken together, these findings suggested that MgIG could attenuate RIPF partly by inhibiting fibroblast differentiation, which was closely related to modulation of the p38MAPK/Akt/Nox4 pathway.