The objective is to evaluate the efficacy and safety of romiplostim for injection in adlut subjects with persistent or chronic primary immune thrombocytopenia (ITP).

Inclusion Criteria:

• Diagnosed with immune thrombocytopenia (ITP) for at least 6 months prior to signing the informed consent form (ICF).
• Subject is ≥ 18 years old while signing the ICF.
• Patients who have no response or relapsed after splenectomy. Or patients who have not been splenectomised and have no response or relapse to at least 1 prior treatment for immune thrombocytopenia (ITP).
• The mean of 3 scheduled platelet counts taken during the screening period must be: < 30 ×10^9/L, with none >35×10^9/L.

Exclusion Criteria:

• Any known history of bone marrow stem cell disorder. Any abnormal bone marrow findings other than those typical of ITP.
• Any active malignancy. If prior history of cancer other than basal cell carcinoma or cervical carcinoma in situ, and no treatment or active disease within 5 years prior to signing the ICF..
• Received hematopoietic growth factors (e.g., granulocyte colony-stimulating factor, macrophage colony-stimulating factor, erythropoietin, interleukin-11) for any reason within 4 weeks prior to signing the ICF.
• Received myeloproliferative leukemia (MPL) stimulation product other than the subject who had suspended recombinant human thrombopoietin (rHuTPO) for injection for 4 weeks before signing ICF.
• Received any anti-malignancy agents (e.g., cyclophosphamide, 6-mercaptopurine, vincristine, vinblastine, Interferon-alfa) for any reason within 8 weeks prior to signing the ICF.
• Received any monoclonal antibody drugs (e.g., rituximab) for any reason within 14 weeks prior to signing the ICF.
• Less than 4 weeks since end of any clinical trials about therapeutic drug or device prior to signing the ICF.
• Pregnant or breastfeeding.
• In the opinions of the principal investigator or investigators, the patients are not suitable for participation in this trial.

Placebo-controlled, Double-blinded then Open-label