The root of Platycodon grandiflorus (PG), with hepatoprotective and anti-oxidation effects, has a long history of being used as food and herbal medicine in Asia. However, the mechanism of PG against non-alcoholic fatty liver disease (NAFLD) is still not clear. The aim of this study was to investigate the mechanism of PG suppressing the development of NAFLD induced by a high-fat diet (HFD) in mice. Male C57BL/6J mice were fed with either a standard chow diet or a HFD, either supplemented with or without PG, for 16 weeks. Serum lipids, liver steatosis, oxidative stress and insulin sensitivity were determined. Expressions or activities of hepatic enzymes in the related pathways were analyzed to investigate the mechanisms. PG significantly reduced HFD-induced hepatic injury and hyperlipidemia, as well as hepatic steatosis via regulating phosphorylation of acetyl-CoA carboxylase (p-ACC) and expression of fatty acid synthase (FAS). In addition, PG ameliorated oxidative stress by restoring glutathione (GSH) content and antioxidant activities, and improved insulin sensitivity by regulating the PI3K/Akt/GSK3β signaling pathway. Our data showed that dietary PG have profound effects on hepatic insulin sensitivity and oxidative stress, two key factors in the pathogenesis of NAFLD, demonstrating the potential of PG as a therapeutic strategy for NAFLD.