The purpose of this study is to investigate the efficacy, safety of Hetrombopag in children with previously treated chronic immune thrombocytopenia who are between 6 and 17 years of age. This is a 2 part study. In part A, patients will receive Hetrombopag for 8 weeks. In part B, all patients will receive Hetrombopag for 24 weeks.

This is a two-part, double-blind, randomized, placebo-controlled, and open-label Phase III study to investigate the efficacy, safety of Hetrombopag in pediatric patients with previously treated chronic ITP. In Part A, patients will receive Hetrombopag for 8 weeks. After completing Part A, patients will begin Part B, in which they will be randomized to receive Hetrombopag or placebo in a 12 week double-blind, placebo-controlled treatment period, following an open-label 12 week treatment period.

Inclusion Criteria:

1. Age ≥ 6 years old and ≤ 17 years old,both sexes.
2. Part A:Confirmed diagnosis of ITP ≥6months; Part B: Confirmed diagnosis of ITP ≥12months;Platelets <30×10^9/L twice in a row,and platelets <30×10^9/L before taking the medicine.
3. Subjects who are refractory or have relapsed after at least one prior ITP therapy.
4. Birth control during and 28 days after the trial.
5. Written informed consent must be obtained from the patient's guardian and accompanying informed assent from the patient (for children over 8 years old).

Exclusion Criteria:

1. No evidence of other causes of thrombocytopenia.
2. Diagnosis as Evans or Wiskott-Aldrich comprehensive.
3. Patients with any prior history of arterial or venous thrombosis, or diagnosis as thrombophilia.
4. Suffering from serious, progressive, uncontrolled kidney, liver, gastrointestinal, endocrine, lung, heart, nervous system, brain, or mental illness.
5. ALT, AST, or ALP> 1.5 x upper limit of normal (ULN), DBLI, or Scr > 1.2 x upper limit of normal (ULN).
6. Active HIV or HCV-Ab positive,HBsAg positive.
7. PT result exceeds normal by more than ±3s, APTT result exceeds normal by more than ±10s
8. Participated in clinical trials of other drugs (received experimental drugs) within 3 months prior to medication.
9. The inestigators determined that other conditions were inappropriate for participation in this clinical trial.