Objective To analyze the muscular MRI imaging features of patients with polymyositis (PM)/dermatomyositis (DM) and explore the clinical application values. Methods The clinical data and MRI images of 90 PM/DM patients were analyzed retrospectively. The imaging features of the spin-echo T1-weightedimages sequence (T1WI), the fast spin-echo T2-weighted images sequence(T2WI) and the shorttime reversal recovery sequence (STIR) were analyzed. Serum creatine kinase(CK), electromyography (EMG), biopsy results and clinical diagnostic results were combined to discuss the advantages and disadvantages of MRI and the clinical application values. Results The sensitivity of MRI was significantly higher than that of CK detection, EMG examination and pathological biopsy(92.22% vs 78.89%, 71.11% and 82.22%,P<0.05). The MRI of PM/DM muscle inflammatory edema showed low signal or equal signal in T1WI and high signal in T2WI and STIR. The MRI of PM/DM myofasciitis showed low signal or equal signal in T1WI, and banded highlight signal in T2WI and STIR. The MRI of PM/DM subcutaneous tissue edema showed low signal or equal signal in T1WI and abnormal high signal in T2WI and STIR. The T1WI, T2WI and STIR of PM/ DM muscle atrophy with fatty infiltration showed spot or flake-like high signal lesions in normal signal muscle group. Conclusion MRI for PM/DM patients can have high sensitivity and specificity, and T1WI signal can clearly show fatty infiltration lesions, and T2WI and STIR imaging have high differential values of inflammatory edema, muscle atrophy and myofascitis, and are suitable to early clinical diagnosis.