Objective To investigate the value of fibrinogen to albumin ratio (FAR) combined with pulmonary embolism severity index (PESI) in the assessment of severity and prognosis of patients with acute pulmonary thromboembolism (APTE). Methods A retrospective study of hospitalized patients with confirmed APTE admitted to the Affiliated Hospital of Southwest Medical University from September 2013 to August 2021, divided into low-risk, intermediate-risk, and high-risk groups according to the Guidelines for the Diagnosis, Treatment and Prevention of Pulmonary Thromboembolism, and divided into survival groups and death groups according to the 30-day prognosis. The general data of all patients and relevant blood laboratory tests within 2 hours after admission were collected to calculate PESI and FAR. FAR and PESI levels were compared in APTE patients with different severity of disease and different prognosis. Independent risk factors for 30-day mortality in APTE patients were analyzed using logistic regression. Subject working characteristic curves were drawn to assess the differences in sensitivity, specificity and area under the curve of FAR, PESI and FAR combined with PESI in predicting 30-day death. Results Total of 235 APTE patients were included, divided into 85 in the low-risk group, 110 in the intermediate-risk group, and 40 in the high-risk group; 192 in the survival group and 43 in the death group according to 30-day survival. The differences in age, albumin (ALB), high-sensitivity troponin, D-dimer, fibrinogen (FIB), FAR, and PESI of APTE patients with different disease severity were statistically significant (P<0.05). FAR increased progressively with increasing severity of disease (P<0.05), and correlation analysis showed a positive correlation between FAR and PESI (r=0.614, P<0.05). Elevated FIB, FAR, PESI and decreased ALB were independent risk factors for 30-day death in patients with APTE (P<0.05). FAR, PESI, and FAR combined with PESI all had predictive value for 30-day death in APTE patients, and FAR combined with PESI predicted the largest area under the 30-day death curve. Conclusions FAR correlated with the severity and prognosis of APTE patients. FAR combined with PESI was more valuable in assessing the 30-day prognosis of APTE patients than FAR alone or PESI alone.