Abstract: 　Objective　To explore the effects of bone marrow mesenchymal stem cells (BMSCs) on viral myositis in mice. Methods　Four-week-old BALB/C male mice were randomly divided into normal control group, myocarditis group, and BMSCs intervention group at different stages (3 days and 2 weeks). The mouse model of viral myocarditis was established by intraperitoneal injection of Coxsackie virus B3. The mice in the intervention group were injected with BMSCs in the tail vein at 3 days and 2nd week after the injection of the virus. Four weeks later, echocardiography was performed, and the pathological integral and collagen volume fraction (CVF) were observed and calculated by light microscopy. The qRT-PCR method was used to detect the mRNA expression of homogenates collagen I (col1α1) and collagen fiber III (col3α1) in myocardial tissue. Results　Compared with the normal control group, the left anterior and posterior wall became thinner, the diameter and volume of the left ventricle at end systolic period was increased; left ventricular ejection fraction (LVEF) and short axis shortening rate (FS) decreased in the myocarditis group. The differences were statistically significant (P all<0.05). The LVEF and FS in each subgroup of the intervention group were better than those of the myocarditis group, and the improvement in the intervention group was more obvious at the 2nd week after the treatment of the myocarditis. The differences were significant (P all<0.05). Light microscope showed that myocardial CVF in myocarditis group was higher than in normal control group, and CVF in intervention group was reduced compared with myocarditis group and CVF in the 2nd week intervention group was lower than that in the 3 day intervention group. The differences were significant (P all<0.05). Compared with the control group, the mRNA expressions of col1α1 and col3α1 in the myocarditis group were increased, and they were lower in the intervention group than in the myocarditis group, and the differences were significant (P all<0.05). Conclusions　BMSCs can reduce the degree of cardiac fibrosis and improve cardiac function in mice with viral myositis, and the intervention effect is better when the virus is infected in the 2nd week.