Objective To investigate the efficacy of conventional rehabilitation alone and conventional rehabilitation combined with aerobic training on muscle strength and function, health condition, and quality of life for patients with stable idiopathic inflammatory myopathy (IIM). Methods This is a historical retrospective cohort study, in which the medical records of patients with IIM, who received the combination of conventional rehabilitative therapy and aerobic training (combined training group [CTG]), from February 2015 to December 2017 were reviewed. Patients with IIM who received conventional therapy alone were matched based on their age, gender, and disease activity as the control group (CG). Scores obtained on manual muscle testing of eight designated muscles (MMT8) was the primary outcome measure, and scores on the myositis Functional Index-2 (FI-2), Health Assessment Questionnaire (HAQ), and 36-item Short Form Medical Outcomes Study Questionnaire (SF-36) at 12 weeks during training were the secondary outcomes. Results Fifty-six patients (28 in the CTG and 28 in the CG) were included in this analysis. Patients in both groups had improved MMT8, FI-2, HAQ, and SF-36 scores after 12 weeks of physical therapy. The CTG had a significantly higher score on the MMT8 and HAQ than the CG in the 12th week. The FI-2 scores were significantly higher in the CTG for the four items ( P < 0.05) of hip flexion, step test, heel lift, and toe lift. SF-36 scores of the CTG were also higher than those of the CG for the five items ( P < 0.05) of physical functioning, general health, vitality, social functioning, and mental health. Conclusions Physical exercise training including conventional rehabilitation and aerobic training improved muscle function, health condition, and quality of life. Conventional rehabilitative training combined with aerobic training achieved better improvement compared with conventional rehabilitation training alone.