Aim To investigate the correlation between systemic immune-inflammation index (SII) and intracranial and extracranial atherosclerotic stenosis. Methods A retrospective analysis was conducted on 489 patients with cerebral infarction admitted to the Department of Neurology of the Affiliated Hospital of Xuzhou Medical University, and all of them were examined by head and neck computed tomography angiography (CTA). Based on the degree of stenosis in these patients, they were classified into three groups:no stenosis group, mild stenosis group (stenosis＜50%), moderate stenosis group (stenosis≥50% and ＜70%), severe stenosis group (stenosis≥70%), and the patient's baseline data, blood lipids, blood routine, etc., were recorded and statistically analyzed. Multivariate binary Logistic regression was used to explore the independent influences on whether stenosis occurred. Multivariate Logistic regression was used to explore the independent influences on the degree of stenosis. The ROC curve was used to explore the efficacy of SII in detecting intracranial and extracranial atherosclerotic stenosis. Results Multivariate binary Logistic regression analysis found that age (OR＝1.185, P＜0.05), SII (OR＝1.482, P＜0.05), gender (OR＝2.721, P＝0.003), hypertension (OR＝3.119, P＜0.001) were independent risk factors for intracranial and extracranial atherosclerotic stenosis. Multivariate Logistic regression analysis showed that hs-CRP (OR＝1.197, P＝0.011), SII (OR＝1.379, P＜0.001) and Hcy (OR＝1.517, P＝0.013) significantly affected the degree of vascular stenosis. ROC curve analysis showed that the area under the curve (AUC) of SII was 0.747, and the best cut-off value of SII was 447.89. Spearman correlation analysis showed that SII level was significantly positively correlated with the severity of intracranial and extracranial atherosclerotic stenosis (r＝0.4,0.482, P＜0.05). Conclusion SII level is positively correlated with the degree of intracranial and extracranial atherosclerotic stenosis in patients with cerebral infarction, and has a certain predictive value for intracranial and extracranial atherosclerotic stenosis.