Objective：To explore the effect of ulinastatin on prevention of acute respiratory distress syndrome (ARDS). Methods: A prospective multicentral cohort study was conducted. The patients from three intensive care units (ICUs) of grade A tertiary hospitals in Beijing and a ICU of grade A tertiary hospitals in Cangzhou from January 2012 to December 2014, included 77 ARDS atrisk patients with ulinastatin treatment and 108 ARDS atrisk patients without ulinastatin treatment (control) were eligible. Both groups received normal treatment; additionally, the intervention group received 600 000 units of ulinastatin via intravenous infusion for 5 days. The control group received the same amount of saline via intravenous infusion for 5 days. Venous blood human neutrophil elastase (HNE) and peptidase inhibitor 3 (PI3) levels were measured on days 1, 3, and 7, respectively. Other outcomes included acute physiology and chronic health evaluation scoring Ⅱ (APACHE Ⅱ), body temperature, respiratory rate, heart rate, mean arterial pressure, white blood cell counts, PaO2/FiO2, ARDS incident, mechanical ventilation time, ICU treatment and hospitalization duration, 28 days mortality.  Results: The PI3 levels showed no statistical difference on day 1, but significant differences on day 3 and day 7 between the two groups (P<0.01). HNE/PI3 ratio showed no statistical difference on day 1, but significant differences on day 3 and day 7 (P<0.05). PaO2/FiO2 was significantly higher in ulinastatin group on day 3 and day 7 (P<0.05). The incident rate for ulinastatin group was 15.58%, lower than that for the control group (33.33%), and the difference was statistically significant (P<0.05). The mechanical ventilation time and ICU treatment time in ulinastatin group was shorter than that in the control group, and the difference was statistically significant (P<0.05). There were no significant effects in other factors. Conclusion: Increased dose of ulinastatin can recover the balance of HNE and its antagonist, lower the HNE’s damage to lungs, and further reduce the ARDS incident rate.