Inclusion Criteria:  Males and non-pregnant females who are 18 years or older (inclusive). Signed informed consent. Subjects are to provide informed consent prior to any study procedures being performed. Consent can be oral if a written consent cannot be expressed. Where it is not practicable to approach a subject highly dependent on medical care, or the subject is not capable of making such a decision, consent will be sought from the legal representative of the subject. Subjects enrolled in the study based on consent by the legal representative will be given the opportunity to provide written confirmatory consent when and if they become able to do so. If the subject declines to confirm consent, they will be withdrawn from the study at the point where they decline consent. Virological diagnosis of severe acute respiratory syndrome coronavirus 2（SARS-CoV-2）infection (documentation of real-time polymerase chain reaction（PCR）or equivalent within the last 72h positive results is available before screening) Hospitalized due to clinical/chest image diagnosis of sever pneumonia. Severe pneumonia defined as dyspnea, hemoglobin oxygen saturation（SpO2）on room air at rest ≤93% or Partial pressure of oxygen（PaO2）/Fraction of inspired oxygen（FiO2）<300 mmHg.  Exclusion Criteria:  Subjects who have renal impairment at screening, defined as an estimate glomerular filtration rate (eGFR) <55 ml/min/Body Surface area（BSA） Subjects requiring extracorporeal membrane oxygenation (ECMO) at screening Subjects who are on invasive mechanical ventilator more than 24 hours Female subjects of child-bearing potential (as judged by the Investigator) who do not agree to remain abstinent or use medically acceptable methods of contraception (e.g., implants, injectable, combined oral contraceptives, intra-uterine devices [Intrauterine devices（IUDs）], double-barrier protection) during the study. Male participants who do not agree to use a condom with spermicide during intercourse (if not surgically sterilized) during the study. Subjects who receive anticoagulants overall (except subtherapeutic doses of heparin which is ≤ 6000 International Unit（IU）twice a day Enoxaparin or equivalence) including but not limited to warfarin, rivaroxaban, apixaban, dabigatran acenocoumarol, fencoumarol, or other parenteral anticoagulants at randomization. Antiplatelets drugs are allowed Subjects who have International Normalized ratio or International normalized ratio（INR）>1.3 Upper limit of normal（ULN）. Subjects who have Absolute Neutrophil Count (ANC) <1,000/μL. Subjects who have platelets count <80,000 /μL. Subjects who have activated Partial Thromboplastin Time (aPTT) >1.5 Upper limit of normal（ULN）. Severe anaemia (haemoglobin < 7.0 g/dL). Bleeding in the past 24 hours requiring blood transfusion. Women who are pregnant or lactating at Screening or planning to conceive (self or partner) at any time during the study, including the follow-up period.  Subjects who have the following chronic organ dysfunction or immunosuppression:  Heart: New York heart association cardiac function IV. Lung: severe lung diseases other than COVID-19 lead to home oxygen therapy. Kidneys: renal impairment requiring chronic dialysis Liver: liver cirrhosis confirmed by biopsy and clear portal hypertension; the upper digestive tract hemorrhage caused by portal hypertension; or previous liver failure/hepatic encephalopathy/hepatic coma. Immune function: IV chemotherapy and radiotherapy within 4 weeks before screening or use of immunosuppressive medication within 2 weeks before screening including but not limited to: tacrolimus, cyclosporin, sirolimus, mycophenolate or azathioprine etc., or subjects with leukemia, lymphoma or acquired immunodeficiency syndrome [AIDS]. Solid organ or bone marrow transplantation within 4 weeks.  The following conditions occurring within 4 weeks prior to screening:  Acute pulmonary embolism Acute coronary syndrome such as myocardial infarction, unstable angina pectoris etc. Known allergy to the active ingredient of STC3141 or its excipients (i.e., phosphate buffer). The investigator consider participation in the study is not in the best interests of the subjects. Prior or ongoing medical conditions, medical history, physical findings, or laboratory abnormality that, in the Investigator's (or delegate's) opinion, could adversely affect the safety of the subject; affect study endpoints or decrease the chance of obtaining satisfactory data required to achieve the objectives of the study. Any other severe or unstable medical condition that, in the opinion of the Investigator or Sponsor, could be expected to progress, recur, or change to such an extent that it could put the subject at special risk, or bias the assessment of the clinical or mental status of the subject to a significant degree. Simultaneous participation in other interventional trials which could interfere with this trial; simultaneous participation in registry and diagnostic trials is allowed.