Compared with placebo, evaluate the effects and safety of Ulinastatin(UTI) added to conventional treatment for ARDS; Evaluate the dose response relationship of Ulinastatin for ARDS.

After signed off the informed consent form by patient or surrogate, and after completing the screening phase, the patients who fulfill the inclusion and exclusion criteria will be randomized 1:1:1:1 to the 4 study arms. All the Patients will receive the study treatment drugs for 7 to 14 days. Use Day 28 of last patient as a cut-off day, Collect the follow up visit data on both Day 28 and Day 90 of other patients．

Inclusion Criteria:

1. Have provided signed written informed consent form (ICF) from the patient or the patient's legal representative;
2. Male or Female patients ≥18 years of age;
3. ARDS defined with using 2012 Berlin Criteria;
4. ARDS diagnosed ≤7 days, and patients should be mechanically ventilated(invasive and or noninvasive mechanical ventilation); 5)100mmhg < PaO2/FiO2 < 250mmhg with CPAP/PEEP≥5 cmH2O;

Exclusion Criteria:

1. Patient with known hypersensitivity to Ulinastatin/adjuvant or patient with allergic constitution;
2. Patients with artificial organs replacement therapy for liver or kidney；
3. Glasgow Coma Scale (GCS)≤8；
4. Cardiogenic pulmonary edema , and the cardiogenic pulmonary edema is as the only or primary reason for respiratory failure;
5. ARDS caused by burning, drowning, poisoning;
6. Presence of severe chronic liver diseases (Child-Pugh score 12-15)，or severe chronic respiratory disease with a PaCO2 > 50 mmHg or the use of home oxygen;
7. Neutrophils<1.5×10^9/L
8. Moribund patients (i.e.,expected to live no longer than 24 hours) or with recent (within 7days or anytime during present hospitalization) cardiopulmonary arrest;
9. Patients need long-term glucocorticoid treatment or need to be treated with immunosuppressive drugs;
10. No intent/unwillingness to follow lung protective ventilation strategy or fluid management protocol;
11. Lung transplant patients;
12. Patients with malignancy, expected to live no longer than 6 months;
13. Pregnant or breast-feeding;
14. Patients who have participated in any clinical study within 3 months prior to the screening;
15. Any condition that in the opinion of the investigator would add the patient's risk or jeopardize the operation of the study.