• Age ≥6 years, ≤18 years, male or female.

• Before enrollment, the subjects have been clinically diagnosed with primary immune thrombocytopenia for no less than three months according to the American Society of Hematology guidelines 2011 Evidence-Based Practice Guideline (Neunert et al. 2011) or the International Consensus Report for the Investigation and Management of Primary Immune Thrombocytopenia (Provan et al. 2010), as applicable locally.

• Patients have failed glucocorticoid therapy (either due to inefficacy, efficacy could not be maintained, or relapse). Patients should have a history of response to previous ITP standard first-line therapy (glucocorticoids and/or intravenous human immunoglobulin) (PLT≥50×10\^9/L);

• Platelet count \<30×10\^9/L within 24 hours before the first administration of the study drug; During the screening period, platelet counts were measured at least 2 times (at least 1 week apart), with an average platelet count \<30×10\^9/L and no platelet count \> 35×10\^9/L.

• ECOG performance status score of ≤2.

• Enrollment of subjects receiving maintenance therapy is permitted, including glucocorticoids (≤0.5 mg/kg of prednisone or equivalent) or TPO receptor agonists. Only one concomitant medication was allowed at the time of enrollment, and the concomitant medication must have been stable for a minimum of 4 weeks prior to the first dose of the study drug.

• For fertile female patients, a negative pregnancy test result is required. Fertile female and male patients must use effective contraception separately during the study and for 4/6 months after the cessation of study drug treatment.

• Subjects comprehensively understand and can adhere to the study protocol requirements and willingly signed the informed consent form.