Background Dysthyroid optic neuropathy (DON) is one of the most serious visual loss threats for patients with Graves’ ophthalmopathy (GO). Barrett’s index (BI) and intracranial-fat prolapse have been used in diagnosing DON. However, these parameters are rarely used in Southeast Asian populations with a variety of cut-off values. Objective To evaluate the performance of BI and fat prolapse in diagnosing of DON, and to study the correlation between their parametric values with visual acuity (VA) and visual field defect (VF). Methods Between January 2011 and December 2020, orbits affected by GO were retrospectively reviewed and classified into 2 groups based on the presence or absence of DON. All orbital-computed-tomography (CT) scans were measured for BI and fat prolapse. Diagnostic performance of BI and fat prolapse was analyzed and evaluated in relation to visual outcome. Results We included orbits with DON (23 orbits) and the absence of DON (61 orbits). BI was significantly higher in patients in the DON group (47.68 ± 12.52%) compared to the absence of DON (37.55 ± 10.88%), p < 0.001. The presence of fat prolapse was significantly higher in the DON group (p = 0.003). BI at 40% provided best diagnostic performance with sensitivity of 78.3%/specificity of 63.9%. The presence of fat prolapse 4.5 mm via the superior-ophthalmic-fissure (SOF) had a lower sensitivity compared with fat prolapse 2.5 mm. Comparison between area under the curve (AUC) of BI and fat prolapse revealed no statistically significant difference (AUC 0.742 and 0.705 in BI and fat prolapse, respectively, p = 0.607). A negative correlation between the BI and fat prolapse with VA and VF was observed (p < 0.001). Conclusion Measurement of BI is a simple diagnostic method for detecting DON in Thai populations. The presence of fat prolapse (2.5 mm) provides a lower sensitivity compared with a BI at 40%. A slightly larger BI or fat prolapse should be suspected of DON for early treatment.