ObjectiveWhether blood urea nitrogen （BUN） can help increase risk stratification of simplified pulmonary embolism severity index （sPESI） in intensive care unit （ICU） still remains unknown. MethodsA total of 250 patients diagnosed as acute pulmonary embolism （APE） at ICU admission from medical information mart for intensive care Ⅲ database （MIMIC） were included in this retrospective study. The 30-day mortality was defined as the primary outcome. The optimal cut-off values of BUN and sPESI were calculated based on the maximum Youden index of receiver operating characteristic （ROC） curves. The Kaplan-Meier curves were used to evaluate the risk stratification ability of BUN， sPESI， and their combinations in the total cohort and subgroups. ResultsBUN and sPESI were both independent predictors for the 30-day mortality. The optimal cut-off value of BUN was 21.5 mg/dL and that of sPESI was 2.5. The 30-day mortality was 41.38% in high-risk group with BUN ≥21.5 mg/dL and sPESI ≥3， which was higher than that in group with sPESI ≥1 （14.28%）. And it was 4.07% in low-risk group with BUN ≤21.5 mg/dL and sPESI ≤2， which was near that of the group with sPESI =0 （3.85%）. ConclusionsBUN can help define a higher risk group with sPESI in APE admitted to ICU， which can contribute to the clinical management.