This is a multicenter, randomized, double-masked, placebo-controlled Phase II/III study in subjects with active thyroid eye disease. Approximately 114 subjects meeting study eligibility criteria will be randomly assigned to IBI311 or placebo on day 1 (D1) in a 2:1 ratio stratified by smoking status

Key Inclusion Criteria:

1. Written informed consent.
2. Male or female subject between the ages of 18 and 80 years, inclusive, at Screening.
3. Weight between 45 and 100 kg (inclusive).
4. Clinical diagnosis of Graves' disease associated with active TED with a CAS ≥ 3 for at least one eye at Screening and Baseline.
5. Moderate-severe active TED (not sight-threatening but has an appreciable impact on daily life), usually associated with at least two of the following: lid retraction ≥2 mm, moderate or severe soft tissue involvement, exophthalmos ≥ 3 mm above normal for race and gender, and/or inconstant or constant diplopia.
6. Female Subjects should be fertile women who are sterile or have a negative blood pregnancy test during the screening period and who agree to take contraceptive measures within 120 days from the screening period to the last medication; Male subjects should agree to use contraception from the screening period to 120 days after the last dose.

Key Exclusion Criteria:

Subjects will be ineligible for study participation if they meet any of the following criteria:

1. At the time of screening, according to the subject's chief complaint or medical record, the time of first symptoms of active thyroid eye disease appeared in > 270 days;
2. Optimal corrected vision loss due to optic neuropathy, defined as two lines of vision loss due to optic neuropathy within the past 180 days, new visual field defects, or color vision impairment secondary to optic nerve involvement;
3. Corneal ulcers with no relief after treatment were determined by the investigator;
4. Baseline CAS decreased by more than 2 points compared with screening;
5. At any time prior to baseline or during the study period planned to receive orbital radiation therapy or TED specific surgery, including orbital decompression, strabismus surgery, and eyelid retraction correction;
6. Poor thyroid function control was defined as free triiodothyronine (FT3) or free tetriodothyronine (FT4) deviated more than 50% from the normal reference range of the local research center laboratory during screening;