Objective To investigate the effects of lentiviral-mediated Fg12 gene silencing on Th1 / Th2 drift and the Th17 / Treg balance in rats with autoimmune myocarditis. Methods Six-week-old SD male rats were randomly divided into four groups: control (NC), model (Model), vehicle lentivirus (Vehicle), and Fgl2-RNAi lentivirus (RNAi group) groups with 10 rats in each group. A rat model of autoimmune myocarditis was established by injection of porcine myoglobin, and then the Fgl2-RNAi lentivirus was injected into the tail vein of rats. VEDs, LVEDd, LVEF, and FS were detected in each group after 28 days of immunization. HE staining was used to observe pathological changes of the rat myocardium. IFN-γ, IL-4, IL-17, and TGF-β mRNA and protein expression was detected by qRT-PCR and Western blot, respectively. Results After 28 days of modeling, LVEDs and LVEDd in the RNAi group were significantly lower than those in the Model group, whereas LVEF and FS in the RNAi group were significantly higher than those in the Model group. The inflammation scores of Model, Vehicle, and RNAi groups were significantly higher than that of the NC group, while the inflammation score of the RNAi group was significantly lower than that of the Model group. The expression levels of IFN-γ and IL-17 mRNA and protein in myocardial tissue of the RNAi group were significantly lower than those in the Model group, while the mRNA and protein expression levels of IL-4 and TGF-β were significantly higher than those in the Model group. Conclusions The Fg12 gene participates in the development of autoimmune myocarditis by regulating Th1 / Th2 drift and the Th17 / Treg balance. Fg12 gene silencing significantly improves cardiac functions and reduces myocardial inflammation in rats with autoimmune myocarditis.