Objective To investigate the value of magnetic resonance imaging (MRI) T₂-mapping in evaluating the activity of Graves ophthalmopathy (GO). Methods A total of 64 patients with GO in the Department of Endocrinology, the First Affiliated Hospital of Chongqing Medical University from July 2019 to January 2021 were collected. Simple random grouping was performed by computer, with 49 cases as observation subjects, and 15 patients for diagnostic test. According to clinical activity score (CAS), 49 GO patients were divided into active group (CAS ≥3 points, 48 eyes) and inactive group (CAS <3 points, 50 eyes). Normal control group (NC group) included 31 patients (62 eyes). All subjects underwent 3.0T orbital MRI T₂-mapping. Measuring the T₂ relaxation time (T₂RT) of superior rectus, inferior rectus, medial rectus, and lateral rectus on five layers behind the eyeball on T₂-mapping coronal images, and select the maximum value of T₂RT in the five layers for each extraocular muscle to represent the T₂RT of this extraocular muscle. Finally, select the maximum T₂RT values of the four extraocular muscles, expressed as extraocular muscle maximum T₂RT. Compare the differences of the above 5 indicators (superior rectus T₂RT, inferior rectus T₂RT, medial rectus T₂RT, lateral rectus T₂RT, extraocular muscle maximum T₂RT) between active group, inactive group and NC group. ROC curve was used to analyze the diagnostic value of the above 5 indicators for GO activity assessment, and the diagnostic threshold was obtained. Then, another 15 GO patients were performed for diagnostic tests evaluation to determine the indicators of high diagnostic efficacy and the threshold of diagnostic activity. Results The T₂RT of all extraocular muscles in active group were significantly higher than those in inactive group and NC group, the difference was statistically significant (P<0.001). The threshold value of the five indicators were obtained by ROC curve analysis. The maximum T₂RT cut-off values of superior rectus muscle, inferior rectus muscle, medial rectus muscle, lateral rectus muscle and extraocular muscles for judging activity were 80.200 ms, 97.045 ms, 94.355 ms, 85.750 ms and 101.385 ms respectively. Another 15 GO patients were performed for diagnostic tests, the indexes with relatively high sensitivity, specificity, positive predictive value and negative predictive value were inferior rectus T₂RT and extraocular muscle maximum T₂RT, the cut-off values of GO activity were 97.045 ms and 101.385 ms, respectively; the sensitivity were 91.7% and 93.8%, respectively; the specificity all were 80.0%. Conclusions MRI T₂-mapping sequence has a good value in assessment of GO activity. The inferior rectus T₂RT and extraocular muscle maximum T₂RT can be choosed to evaluate the activity of GO.