Polymyositis (PM) is a rare neuromuscular phenotype of chronic graft-versus-host disease (cGVHD). Although glucocorticoids have been shown to be effective for PM treatment, most people have poor treatment response and poor prognosis. Herein, we reported a case of thalassemia in a 6-year-old boy who received allogeneic hematopoietic stem cell transplantation (HSCT) and consequently developed severe chronic GVHD complicated with polymyositis. He was then given high-dose corticosteroid therapy, including tacrolimus, ruxolitnib, and rituximab. Twenty-three months after transplant, creatine kinase levels return to normal range, the MRI showed that the original muscle edema signal was significantly improved. In addition, the patient’s muscle weakness continued to improve, and his overall condition was good. This report suggests that glucocorticoids combined with immunosuppressants may be effective agist polymyositis. Rituximab and ruxolitinib may be a good choice in treating polymyositis.