Inclusion Criteria:

• * Diagnosis of myelofibrosis (primary, post-polycythemia vera, or post-essential thrombocythemia).
• * Relapsed/refractory to a marketed (FDA approved) JAK inhibitor.
• * At least 18 years of age.
• * ECOG PS 0, 1, or 2.
• * Expected life expectancy is greater than 24 weeks.

Exclusion Criteria:

• * Any chemotherapy, immunomodulatory therapy, immunosuppressive therapy, corticosteroids, or growth factor treatment within 14 days prior to initiation of study drug.
• * Major surgery or radiation therapy within 28 days prior to initiation of study drug.
• * With suspected allergies to jaktinib or its excipient.
• * Another clinical trial of a new drug or medical instrument within 3 months before screening.
• * Females who are pregnant, currently breastfeeding, planning to become pregnant.
• * Unable to adopt effective contraceptive methods during the study.