Objective:To evaluate the relationship between systemic immune-inflammatory index (SII) and prognosis of acute respiratory distress syndrome (ARDS).Methods:ARDS patients from the I Medical Information Mart for Intensive CareⅢ were selected as the study objects. Patients were followed up for one year. The primary endpoint was the 30-day mortality rate, and secondary endpoints were the 90-day and one-year mortality rates. Cox proportional hazard regression analysis was used to assess SII as an independent risk factor for ARDS patients, with propensity score matching to control for confounding factors.Results:A total of 723 ARDS patients were included in this study. Patients with SII≥ 3655 had older age, lower SpO 2 levels, and higher simplified acute physiology scoreⅡ (SAPSⅡ) and sequential organ failure assessment (SOFA) scores compared to those with SII <3655. Additionally, the 30-day, 90-day, and one-year mortality rates were higher in patients with SII ≥3655. Cox proportional hazard regression analysis showed that high SII level was an independent risk factor for the prognosis of ARDS at 30 days ( HR=1.68, 95% CI: 1.19-2.36, P=0.0028), 90 days ( HR=1.46, 95% CI: 1.07-1.99, P=0.0170), and one year ( HR=1.34, 95% CI: 1.01-1.77, P=0.0425). Propensity score matching analysis further confirmed the relationship between SII and the prognosis of ARDS patients. Conclusions:SII, as a simple and easily measurable index, is an independent risk factor for the prognosis of ARDS patients.