Objective To establish a model of radiation-induced myocardial fibrosis in mice, and study the effect of TGF-β/SMAD pathway on radiation-induced myocardial fibrosis in mice and the protective effect of pirfenidone capsules (PFD) on radiation-induced myocardial fibrosis. Methods Male C57BL/6J mice were randomly divided into control group, irradiation group, and drug intervention groups (low-dose PFD + irradiation group and high-dose PFD + irradiation group). After radiation exposure, the control group and the irradiation group were given sodium carboxymethyl cellulose by gavage, while the low-dose PFD + irradiation group and the high-dose PFD + irradiation group were given PFD at 150 and 300 mg/kg daily by gavage for 12 consecutive weeks, respectively. The body weights of mice were measured and recorded weekly. The pathological changes of heart tissues in mice were observed by H&E and Masson’s trichrome staining. The expression levels of TGF-β, SMAD2, and SMAD3 mRNAs in mouse heart tissues were detected by RT-PCR. The expression levels of TGF-β, SMAD2, and SMAD3 proteins in mouse heart tissues were detected by Western blot. The expression levels of CK and CK-MB in mouse heart tissues were detected by blood biochemical tests. Results H&E and Masson’s trichrome staining showed severe myocardial fibrosis in the irradiation group compared with the control group. Compared with the irradiation group, the two groups with drug intervention showed reduced heart disease and myocardial fibrosis. RT-PCR and Western blot showed that the expression levels of TGF-β, SMAD2, and SMAD3 were up-regulated in the irradiation group compared with the other three groups (P < 0.05). The expression levels of TGF-β, SMAD2, and SMAD3 were down-regulated in the two groups treated with PFD compared with the irradiation group (P < 0.05). The expression levels of CK and CK-MB in mouse heart tissues were down-regulated in the two groups treated with PFD compared to irradiation group (P < 0.05). Conclusion Radiation has long-term effects on normal heart tissue, causing myocardial damage and promoting myocardial fibrosis. The up-regulation of TGF-β/SMAD pathway is related to the formation of radiation-induced myocardial fibrosis. PFD can mitigate the progression of myocardial fibrosis and protect heart tissue by down-regulating TGF-β/SMAD pathway.