Inclusion Criteria:  Subjects voluntarily sign the informed consent form (ICF); Age ≥ 18 years, either male or female; Subjects diagnosed with a PMF according to World Health Organiztion (WHO) criteria (2016 Edition), or patients diagnosed with a Post-PV-MF or Post-EF-MF according to International Working Group for Myeloproliferative Neoplasms Research and Treatment (2007 IWG-MRT) criteria; Subjects with intermediate-2 or high-risk myelofibrosis, or Intermediate-1 myelofibrosis with symptoms according to the Dynamic International Prognostic System (DIPSS) scoring system;  Subjects are relapsed/refractory to Ruxolitinib:  Relapsed defined as Ruxolitinib treatment for ≥ 3 months, following an initial response, regrowth to < 10% spleen volume reduction (SVR) by MRI or < 30% decrease in spleen size by palpation from baseline; Refractory defeined as Ruxolitinib treatment for ≥ 3 months observed inadequate efficacy response: < 10%volume SVR by MRI or < 30% decrease in spleen size by palpation from baseline. Subject has a measurable splenomegaly: spleen volume of ≥ 450 cm3 by MRI/CT and ≥ 5 cm below left costal margin by palpation spleen measuring; Expected life expectancy is greater than 24 weeks; Eastern Cooperative Oncology Group (ECOG) performance Score 0-2; Laboratory examination within 7 days before the randomization, fulfilling the following criteria:  Neutrophil count ≥ 0.75 x 109/L, platelet count ≥ 75 x 109/L; Peripheral blood blasts ≤ 10%; ALT and AST≤ 3 ULN, DBIL ≤ 2.0 ULN; Serum creatinine ≤ 2.0 ULN.  Exclusion Criteria:  Subjects who have been previously exposed to Janus kinase (JAK) inhibitors other than Ruxolitinib for a total of> 2 weeks; Subjects who have taken Ruxolitinib or other JAK inhibitor within 1 week prior to screening; Subjects with any significant clinical and laboratory abnormalities which may affect the safety evaluation, such as uncontrolled diabetes, uncontrolled hypertension after taking two or more hypotensive drugs or peripheral neuropathy; Subjects with congestive heart failure (NCI-CTCAE V5.0) Class II or above, uncontrolled or unstable angina or myocardial infarction, cerebrovascular accident, or pulmonary embolism within 6 months prior to screening; Subjects who have a history of chronic or recurrent respiratory diseases, such as: chronic obstructive pulmonary disease, recurrent lung infections, etc., or have a history of lung infections within 3 months before screening, or currently have upper respiratory tract infections that have not recovered; Subjects who have not fully recovered from surgical operation within 4 weeks prior to screening; Subjects suffering from arrhythmia and requiring treatment, or QTcB > 480ms at screening; Subjects with clinical symptoms of active bacterial, viral, parasitic or fungal infections requiring treatment at screening; Subjects who had undergone splenectomy, or received radiotherapy to the spleen within 6 months before screening; Subjects with known human immunodeficiency virus (HIV), known active infectious Hepatitis B (HepB), and/or known active infectious Hepatitis C (HepC); Female subjects who are pregnant, currently breastfeeding, planning to become pregnant; Subjects who had experienced malignant tumors (except for adequately treated local basal cell or squamous cell carcinoma of the skin and cervical carcinoma in situ that have been cured) or in combination with other serious diseases within the past 5 years; Subjects who have participated in another clinical trial of a new drug or medical instrument within 1 month before screening. Subjects who have any other conditions that are not specified in the protocol but the investigator believes that they are not suitable for inclusion in this trial.