Persistent hyperglycemia increases inflammation response, promoting the development of myocardial fibrosis. Based on our previous research that exercise and metformin alone or their combination intervention could attenuate myocardial fibrosis in db/db mice, this study aimed to further explore the underlying mechanisms by which these interventions attenuate myocardial fibrosis in early diabetic cardiomyopathy. Forty BKS db/db mice were randomly divided into four groups. Diabetic db/db mice without intervention were in the C group. Aerobic exercise (7–12 m/min, 30–40 min/day, 5 days/week) was performed in the E group. Metformin (300 mg·kg⁻¹·day⁻¹) was administered in the M group. Exercise combined with metformin was performed in the EM group. Ten wild-type mice were in the WT group. All interventions were administered for 8 weeks. Results showed that the expression levels of α-SMA, Collagen I, and Collagen III were increased in 16-week-old db/db mice, which were reversed by exercise and metformin alone or their combination intervention. All interventions attenuated the level of TGF-β1/Smad2/3 pathway–related proteins and reduced the expression of inflammatory signaling pathway–regulated proteins TNF-α, p-IκBα/IκBα, and p-NF-κB p65/NF-κB p65 in db/db mice. Furthermore, metformin intervention inhibited HNF4α expression via AMPK activation, whereas exercise intervention increased the expression of IL-6 instead of activating AMPK. In conclusion, exercise and metformin alone or their combination intervention inhibited the TGF-β1/Smad pathway to attenuate myocardial fibrosis by reducing NF-κB–mediated inflammatory response. The anti-fibrotic effects were regulated by metformin-activated AMPK or exercise-induced elevation of IL-6, whereas their combination intervention showed no synergistic effects.