Objective To investigate the protective effect of metformin (Met) on acute fulminant hepatitis induced by concanavalin A (ConA) in mice and explore its mechanism. Methods Twenty-four mice were randomly divided into normal group (NC), ConA group, and Met group, with 8 mice in each group. The latter two groups respectively were gavaged with 0.2 mL normal saline and metformin (100 mg/kg) for 5 days, followed by tail vein injection of 0.1 mL ConA (25 mg/kg) to establish the acute fulminant hepatitis model, and all the mice were sacrificed 18 hours later. The serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total bilirubin (TB) were detected; the pathological changes of mouse liver tissue were observed with HE staining; the macrophage infiltration in liver tissue was detected with immunohistochemistry. The mRNA of IL-1β, IL-6 and TNF-α in liver tissue were tested with real time quantitative PCR. The number of total white blood cells (WBC) and lymphocytes in the peripheral blood were recorded and the frequency of Th17 cells in the spleen was detected by flow cytometry. The expression of apoptosis protein caspase-3 in liver tissue was observed with immunofluorescence histochemistry and the expression of AMPK and phosphorylated AMPK (p-AMPK) were detected by Western blot analysis. Results Compared with the ConA group, the Met group showed significantly decreased serum ALT, AST and TB, improved liver tissue pathology, decreased macrophage infiltration and increased content of peripheral total WBCs and lymphocytes, as well as decreased frequency of Th17 lymphocytes in the spleen. The expression of IL-1β, IL-6 and TNF-α and apoptosis also decreased in this group, along with the increased expression of p-AMPK. Conclusion Met has a significant protective effect on acute fulminant hepatitis mice, and its mechanism may be related to the activation of AMPK signal, thus reducing the frequency of Th17 lymphocytes and alleviating the infiltration of hepatic inflammatory cells and the production of pro-inflammatory cytokines.