Objective To observe the clinical values of magnetic resonance imaging (MRI) in the diagnosis of polymyositis (PM) and dermatomyositis (DM). Methods 76 cases of myopathy patients suspected PM/DM were selected for the study and were given the MRI, electromyography (EMG) and muscle biopsy. Serum creatine phosphokinase (CK) level was measured, and the skin damages were observed. The sensitivity, specificity, accuracy, positive and negative predictive values of MRI in the diagnosis PM/DM were evaluated based on the combined results of EMG, muscle biopsy, CK and skin damages, and the MRI findings of PM/DM patients were observed. Results Among the 76 cases of subjects, 58 cases were positive and 18 cases were negative by comprehensive diagnosis. The sensitivity, specificity, accuracy, positive predictive value and negative predictive value of MRI in the diagnosis of PM/DM were 81.03%, 94.44%, 84.21%, 97.92% and 60.71% respectively. There were 47 cases (61.84%) of muscle inflammatory edema, 20 cases (26.32%) of subcutaneous connective tissue edema, 32 cases (42.11%) of myofascitis and 5 cases (6.58%) of fat infiltration atrophy. Conclusion The MRI diagnosis for PM/ DM has high sensitivity, specificity, accuracy and obvious image features, and it is noninvasive examination with good repeatability, and it has some help in the early detection of lesion sites and the guidance of follow-up pathological biopsy.