Inclusion Criteria:  Voluntarily signed informed consent; ≥18 years old; Patients developed ARDS in the preceding 72h; The total scores of the Murray lung injury scores were greater than 6 （When lung compliance was not measured, the total scores of the other three scores were greater than 4）;  Exclusion Criteria:  Pregnancy or lactation; ARDS was diagnosed for more than 72 hours; Sivelestat sodium was used for treatment prior to enrollment; Patients with more than 3 extra-pulmonary organ injuries/failure; Severe chronic liver disease (Child-pugh grade C); Previous history of allergy to sivelestat sodium or any of its ingredients or preservatives; Patients whose primary disease cannot be effectively controlled; Patients judged by the investigator to be unsuitable for participation in this study.