Objective To explore the value of 3.0T MRI in the diagnosis of polymyositis (PM) and dermatomyositis (DM). Methods The clinical data of 17 PM cases and 22 DM cases admitted to our hospital from January 2015 to July 2017 were analyzed retrospectively. All patients were given 3.0T MRI examination, and their images were analyzed, and the results were compared with results of serum muscle enzymes markers and muscle biopsy. Results Among 39 PM and DM patients, there were 36 cases of muscular inflammatory edema, 11 cases of myofascial thickening (myofascitis), 18 cases of subcutaneous soft tissue edema and 8 cases of muscular atrophy and fat infiltration. Patients with myofasciitis and subcutaneous soft tissue edema tended to have muscular inflammatory edema. In inflammatory edema area of muscle, there was equal signal on T1WI, high signal on T2WI and high signal on STIR. In the intermuscular space of myofascialitis, there was equal signal on T1WI, high signal on T2WI and high signal on STIR. In the subepithelial fat of subcutaneous soft tissue edema, there was low signal on T1WI showed, slightly high signal on T2WI and high signal on STIR. In the area of muscle atrophy and fatty infiltration, there was short signal on T1WI and long signal on T2WI. There were significant differences between the results of 3.0T MRI scan and the results of serum muscle enzymes markers (P<0.05). There was no significant difference between the 3.0T MRI scan results and muscle biopsy results (P>0.05). Conclusion 3.0T MRI has important application value in the diagnosis of PM and DM. Muscular edema is the most common manifestation of 3.0T MRI in PM and DM patients.