Aim To explore the effect of metformin (Met) regulating macrophage phenotype through adenosine monophosphate-activated protein kinase (AMPK)/signal transducer and activator of transcription 3 (STAT3) signal pathway on the formation of atherosclerosis (As) in mice. Methods RAW264.7 macrophages were cultured in vitro, lipopolysaccharide was used to promote macrophage differentiation, and Met was used to stimulate cells. The proportion of macrophages with different phenotypes (CD86, CD206) was detected by flow cytometry. The protein expression levels of inducible nitric oxide synthase (iNOS), arginase-1 (Arg-1), AMPK, phosphorylated AMPK (pAMPK), STAT3 and phosphorylated STAT3 (pSTAT3) were detected by Western blot. ApoE－/－ mice were fed with high-fat diet to construct As model. Mice in the experimental group (Met group) were given Met intragastric intervention, and mice in the control group (CTL group) were given the same volume of normal saline intragastric intervention. After 3 months, the whole length of the mouse aorta (from the brachiocephalic trunk to the bilateral iliac arteries) and the aortic root were extracted and stained with oil red O and immunofluorescence respectively to evaluate the lipid deposition of the aorta and the different phenotypic proportion of macrophages in the lipid plaque. The large artery proteins were extracted, and the expression levels of AMPK, pAMPK, STAT3 and pSTAT3 in related signal pathways were verified by Western blot. Results Cell experiments showed that after Met stimulation, the proportion of M1 macrophages decreased significantly, the proportion of M2 macrophages increased significantly (P＜0.05), AMPK activity increased significantly, and STAT3 activity decreased significantly. Animal experiments showed that three months after modeling, oil red O staining showed that the lipid depositions in the whole length of large artery and aortic valve in Met group were significantly lower than those in CTL group (P＜0.05); immunofluorescence staining showed that the proportion of M1 macrophages in lipid plaque in Met group was significantly lower than that in CTL group, while the proportion of M2 macrophages was significantly higher than that in CTL group (P＜0.05). Western blot experiment showed that AMPK activity increased significantly and STAT3 activity decreased significantly in Met group compared with the CTL group (P＜0.05). Conclusion Met inhibits the activity of STAT3 by activating AMPK, regulates the phenotypic differentiation of macrophages in plaque and inhibits the formation of As in mice.