Objective To investigate the quantitative value of the signal intensity ratio of extraocular muscle and ipsilateral white matter measured by MRI for the evaluation of activity in thyroid-associated ophthalmopathy. Methods A total of 129 patients and 245 eyeballs with thyroid-associated ophthalmopathy were enrolled in this study and this 245 eyeballs were set as thyroid-associated ophthalmopathy group(TAO group). There were 10 patients with newly diagnosed Graves′ disease and in the same period and these 20 eyeballs were set as graves′ disease group(GD group). 32 normal people from annual physical test excluded thyroid and eye diseases and their 64 eyes were selected randomly for the normal control group (NC group). The signal intensity of the extraocular muscle and the ipsilateral white matter on the MRI images were measured, while did exophthalmos and the width of the inner fat of eyeballs(FWs)measurements in the same time. Results SIR, FWs, and exophthalmos of TAO group were higher than those of the other 2 groups[SIRs: 1.71(1.40, 2.10)vs 1.26(1.22, 1.34)and 1.23(1.14, 1.32); FWs: 8.04(6.70, 8.71)mm vs 6.16(4.86, 7.08)mm and 6.93(6.41, 7.65)mm, exophthalmos: 20.10(18.56, 22.15)mm vs 15.40(14.87, 16.60)mm and 14.73(13.40, 16.07)mm, all P 0.05). The SIRs and CAS had a positive correlation(r=0.580, P=0.000), through the receiver operating characteristic curve(ROC)we get the best diagnostic performance of TAO activity when the SIR≥1.56(sensibility=65.6%, specificity=89.1%, AUC=0.815, P=0.000). Conclusion The signal intensity ratio of extraocular muscle and ipsilateral white matter may discriminate the activity of TAO early as a quantitative indicator, reflecting its efficacy, and is worth clinically generalizing. (Chin J Endocrinol Metab, 2018, 34: 106-111) Key words: Thyroid-associated ophthalmopathy; Signal intensity ratio of extraocular muscle to ipsilateral white matter; Activity evaluation; Quantitative indicator