The object of this study is to observe the safety of umbilical cord mesenchymal stem cell therapy for acute respiratory distress syndrome, consisting with two phases.

The secondary objectives are to observe the preliminary efficacy of umbilical cord mesenchymal stem cell therapy for acute respiratory distress syndrome. Phase One is an open-label, dose-escalation clinical study. Following safety and tolerability conclusions from Phase One, and upon evaluation and approval by the data safety committee and ethics committee, conduct Phase Two study, which will be a randomized, double-blind, placebo-controlled study. Dosage selection will be based on the safest and most effective dosage determined from Phase One results.

Inclusion Criteria:

1. Age greater than 18 years, regardless of gender;
2. Meeting the ARDS Berlin diagnostic criteria;
3. Definite diagnosis within 72 hours;
4. P/F < 150mmHg;
5. Understanding and signing informed consent.

Exclusion Criteria:

1. Women of childbearing potential with a positive serum pregnancy test before medication, pregnant women, or lactating women;
2. ARDS directly caused by physical or chemical factors;
3. Moderate to severe liver damage (Child-Pugh score >12);
4. Chronic heart failure, New York Heart Association functional class IV;
5. Severe kidney disease undergoing renal replacement therapy;
6. Severe lung disease, Grade III or IV pulmonary hypertension, receiving oxygen therapy or ventilator support for more than one month in the six months prior to screening, end-stage lung disease, or severe physical limitations due to chest wall deformity;
7. Immunodeficiency, receiving immunosuppressive therapy within 2 weeks (except for low-dose corticosteroids), or with lymphoma, leukemia, or acquired immune deficiency syndrome; history of organ transplantation, bone marrow transplantation, or autologous hematopoietic stem cell suppression;
8. Expected survival time of less than 48 hours due to terminal illness;
9. Patients diagnosed with deep vein thrombosis (DVT) or pulmonary embolism (PE) in the past three months;
10. Patients receiving ECMO therapy;
11. Hepatitis B, hepatitis C, syphilis, or HIV infection.
12. eGFR < 30ml/min/BSA;
13. ALT > 5 × ULN;
14. Absolute neutrophil count < 1500/μL;
15. Subjects who have participated in other clinical studies within the past month (excluding those who have not received intervention), or are currently participating in other experimental treatments;
16. Subjects deemed unlikely to benefit from the study by the investigator, or deemed unsuitable for participation in this study.