Objective To observe the changes of CD4⁺, CD8⁺T cell proportions and the level of cell surface programmed death-1 (PD-1) in the peripheral blood mononuclear cells (PBMC) of patients with thyroid-associated ophthalmopathy (TAO), and to explore their roles in the pathogenesis and diagnosis of TAO. Methods The clinical data of 21 TAO patients, 21 Graves disease (GD) patients without ophthalmopathy and 20 healthy volunteers were collected. The proportions of CD4⁺, CD8⁺T cells in PBMCs and cell surface PD-1 expressions were tested by flow cytometry and were compared between different groups; and their correlation with disease course, thyroid functions, thyroid associated antibodies and severity of TAO were analyzed. Results (1)The proportions of CD4⁺, CD8⁺T cells in PBMCs of TAO patients ([35.9±14.5]% and [22.2±8.4]%, respectively) and GD patients ([33.1±13.0]% and [18.6±9.2]%, respectively) were both significantly higher than those in the healthy volunteers ([17.4±7.4]% and [7.7±4.8]% respectively, both P<0.01). (2) The expression of PD-1 on CD4⁺T cells in TAO and GD patients were (8.3±6.4)% and (28.0±26.6)%, respectively, with both being significantly lower than those in the healthy volunteers ([52.2±28.6]%, P<0.01 and P<0.05, respectively), and that in TAO patients was significantly lower than that in GD patients (P<0.05). The expression of PD-1 in CD8⁺T cells was significantly lower in TAO patients than that in the healthy volunteers ([12.7±13.4]% vs [38.2±24.2]%, P<0.01). (3) Correlation analysis showed that there was a positive correlation between disease course and CD8⁺T cell proportion in PBMCs of TAO patients (r=0.478, P<0.05). (4) There was no significant difference in CD4⁺,CD8⁺T cells proportions or their PD-1 expressions between active and inactive, mild and moderate-to-severe TAO patients (P>0.05). Conclusion The aberrant proportions of CD4⁺, CD8⁺T cells in PBMCs and cell surface PD-1 expression may participate in the autoimmune process of TAO.