【Abstract】 Objective To investigate whether SPA levels in bronchoalveolar lavage fluid (BALF) correlates with the severity of ARDS in adult. Methods From May 2012 to January 2018, 151 adult patients (18 to 75 years old) with ARDS requiring invasive mechanical ventilation from RICU in our hospital were screened in our study. Finally, 75 patients were enrolled. According to whether survived in 28 days after invasive mechanical ventilation intervention, patients were divided into two groups, nonsurvival group (n= 48) and survival group (n= 27). Demographic characteristics, clinical data and the content of SPA in BALF were recorded and measured on day 0. Correlation analysis was used to evaluate the relationship between SPA level in BALF and PaO2/FiO2. Results Our data showed that the significant differences in body mass index (BMI), APACHEII score, PIP, PEEP, WBC, NS, PCT, PaO2/FiO2 and SPA in BALF were found between survival group and nonsurvival group. Strong correlation between SPA level in BALF and PaO2/FiO2 was observed (R2= 0531). Conclusion The general condition of ARDS patients in nonsurvival group was worse than survival group. Correlation between SPA level in BALF and PaO2/FiO2 was good. Therefore, our results indicated that SPA level in BALF could possibly be used as a predictor of the severity of ARDS in clinical practice. However, a multiple center study with larger sample size should be carried out to further assess the value of it in future.