Inclusion Criteria:
1\. Aged after 18 years (18 is included). 2. Locally advanced or metastatic differentiated thyroid cancer (papillary, follicular, Hurthle cells, poorly differentiated carcinoma). At least one measurable lesion (helical CT scan long diameter ≥10mm, meet the requirements of the standard Response Evaluation Criteria In Solid Tumors（RESCIST） version 1.1).
3\. ECOG-PS score 0-2 4. Life Expectancy of at least 12 weeks 5. Subjects must be 131I-refractory / resistant as defined by at least one of the following.
1. Lesions that do not demonstrate iodine uptake on any radioiodine scan.
2. Subjects received a single radioactive iodine therapy within 12 months (≥ 3.7 Giga Bequerel（GBq）\[≥ 100 millicurie（mCi）\]) and target lesion disease progression.
3. Every two radioactive iodine treatment interval \<12 months, doses ≥ 3.7 GBq \[≥100mCi\], disease progress more than 12 months after at least once iodine therapy.
4. Received a total dose of radioactive iodine therapy ≥ 22.2 GBq (≥ 600 mCi). 6. Have the required screening laboratory values.
Exclusion Criteria:
1. Other thyroid cancer histological subtypes (such as medullary carcinoma, lymphoma or sarcoma).
2. Any active autoimmune disease or history of autoimmune disease and expected recurrence (including but not limited to autoimmune hepatitis, interstitial pneumonia, uveitis, enteritis, hypophysitis, vasculitis, nephritis); subjects with skin diseases that does no need systemic treatment, for example, leukoderma, psoriasis, alopecia, those with controlled type I diabetes by insulin or those with asthma that has been completely resolved in childhood and with no need of any intervention can be enrolled; while subjects with asthma who need bronchodilator for medical intervention cannot be enrolled.
3. Use of strong CYP3A4/CYP2C19 inducers, including rifampicin (and its analogues) and St. John's Wort, or strong CYP3A4/CYP2C19 inhibitors within two weeks prior to signing informed consent form.
4. Previous treatment with other immune checkpoint inhibitors (include PD-1 antibody or other immunotherapy against PD-1/PD-L1).
5. Known history of serious allergy to any monoclonal antibody or Apatinib.
6. Inability or unwilling to swallow tablets, malabsorption syndrome or any condition affecting gastrointestinal absorption.
7. Previous or current presence of metastasis to central nervous system.
8. Severe infection within 4 weeks prior to the start of study treatment, including but not limited to hospitalization for infection, bacteremia or complications of severe pneumonia; oral or intravenous therapeutic antibiotics within two weeks prior to the start of study treatment (for example, subjects who are given with preventive antibiotics for prevention of urinary tract infection or exacerbation of chronic obstructive pulmonary disease are eligible for participation in the study).
9. Pregnant or lactating women.
10. Other factors that may affect the study results or lead to forced termination of the study early as judged by investigators, such as alcoholism, drug abuse, other serious diseases (including mental disorders) requiring concomitant therapy, with serious laboratory examination abnormality, with family or social factors, that may affect subject's safety.
