This study evaluates the value of [18F]AlF-NOTA-FAPI-04PET/CT in assessing the disease activity of thyroid eye disease (TED).Twenty-three TED patients and ten healthy controls were prospectively recruited and underwent [18F]AlF-NOTA-FAPI-04 PET/CT scans. TED eyes were classified by Clinical Activity Score (CAS) as active (CAS +) or inactive (CAS-). PET analysis included qualitative and quantitative evaluations. Qualitative analysis categorized TED eyes as positive (FAPI +) or negative (FAPI-), while quantitative analysis including SUVmax and target-to-backgroud ratio (TBR) of each extraocular muscle (EOM). Pathological assessment of 16 EOM samples from 15 TED eyes was used as a gold standard to evaluate the diagnostic accuracy of CAS-driven and FAPI-driven assessments.Eye-based analysis included 42 TED eyes and 20 control eyes. SUVmax did not differ significantly between CAS-/FAPI + (n = 8) and CAS + /FAPI + (n = 17) eyes; though both groups had significantly higher SUVmax than CAS-/FAPI- (n = 16), CAS + /FAPI- (n = 1), and control eyes (n = 20) (all P < .05). Diffuse high FAPI uptake with excellent TBR was observed in pathologically active EOMs and had higher SUVmax and TBR than inactive EOMs (SUVmax: 9.57 ± 2.79 vs. 4.04 ± 1.02, TBR: 2.99 ± 1.00 vs. 1.00 ± 0.35, both P < .001). An SUVmax cutoff value of 5.95 yielded an area under the receiver operating characteristic curve of 0.98 for predicting active disease. Sensitivity and specificity of CAS-driven and FAPI-driven assessments for TED activity were 72.7% and 100%, 100% and 100%, respectively. SUVmax positively correlated with pathological fibroinflammatory score (r = 0.68, P = .004).The diffuse high FAPI uptake with excellent TBR in the EOMs could illustrate the activity of TED, positively correlated with pathological fibroinflammatory scores. These findings suggest that [18F]AlF-NOTA-FAPI-04 PET/CT may serve as a promising reliable non-invasive approach for assessing disease activity in TED, potentially superior to the CAS.© 2024. The Author(s).