Objective:To evaluate the risk and influencing factors of pulmonary embolism in patients with iliac vein compression syndrome (IVCS) and acute iliofemoral vein thrombosis by CT pulmonary angiography combined with CT venography of inferior vena cava.Methods:The data of 166 patients with acute left iliofemoral vein thrombosis diagnosed in the Affiliated Hospital of Jining Medical University from July 2016 to June 2020 were analyzed retrospectively. All patients underwent one-stop CT pulmonary angiography combined with inferior vena cava CT venography. The patients were divided into IVCS group (101 cases) and non-IVCS group (65 cases) according to the presence or absence of IVCS. The general data of the patients, the stenosis rate of left common iliac vein, the presence of inferior vena cava floating thrombosis, the presence of large pelvic collateral veins, the detection of pulmonary embolism and the pulmonary artery obstruction index of the two groups were compared, and multivariate logistic regression and multiple linear regression were used to analyze the influencing factors of the incidence and severity of pulmonary embolism in IVCS group.Results:There were significant differences in the stenosis rate of left common iliac vein [(68±8)% vs (25±14)%, t=-25.300, P<0.001], the incidence of inferior vena cava floating thrombosis [25/101, 31/65, χ2 =9.310, P=0.002], the length of inferior vena cava floating thrombosis [17.2 (10.9, 27.8)mm vs 27.4 (20.1, 55.9) mm, Z=-2.316, P=0.021], the incidence of pulmonary embolism (43/101 vs 41/65, χ2 =6.651, P=0.010) and the pulmonary artery obstruction index [(10.0% (5.0%, 17.5%) vs 22.5% (10.0%, 30.0%), Z=-3.490, P<0.001] between IVCS group and non-IVCS group. In the IVCS group, multiple logistic regression analysis revealed that the stenosis rate of left common iliac vein [β=-1.964, OR(95%CI) 0.140(0.031-0.638), P=0.011] and inferior vena cava floating thrombosis [β=1.212, OR(95%CI) 3.360(1.566-7.209), P=0.002] was independent factors for the occurrence of pulmonary embolism. Multiple linear regression showed that the influence of inferior vena cava floating thrombosis on the pulmonary artery obstruction index was statistically significant (b=0.352, t=2.410, P=0.021). Conclusion:The incidence and severity of pulmonary embolism in patients with IVCS and acute left iliofemoral vein thrombosis are lower than those without IVCS, and the presence or absence of inferior vena cava floating thrombosis is an important factor affecting the severity of pulmonary embolism.