Blood-brain barrier (BBB) damage plays an important role in overall brain injury following acute ischemic stroke (AIS). We investigated the potential utility of serum occludin, a BBB damage biomarker, in predicting the severity of AIS, hemorrhagic transformation (HT) and patient prognosis. A total of 243 patients, suspected of suffering an AIS and admitted to the emergency room at Xuanwu Hospital between November 2018 to March 2019, were enrolled in this study. Serum occludin levels were measured by enzyme linked immunosorbent assay and clinical data were collected from each patient. Receiver operating characteristic curves (ROC) were used to analyze the relationship between serum occludin and AIS. Multiple logistic regression analysis was used to analyze the relationship between serum occludin and stroke prognosis. Serum occludin levels were significantly elevated in acute stroke cases compared with those with stroke-like symptoms (P<0.001). In the moderate and severe cerebral infarction (CI) groups, serum occludin levels were significantly higher than those in the mild CI group (P<0.001). Patients with HT had higher occludin levels than non-HT patients (P<0.05). In addition, serum occludin level of patients with poor prognosis was significantly higher than that of the patients with good prognosis for non-reperfusion therapy. The ROC curve showed that serum occludin could reasonably predict HT and poor prognosis. Moreover, serum occludin were independently associated with 90-day poor prognosis. These findings suggest that the serum occludin levels could be used to identify early acute stroke cases and may predict the severity of AIS and HT as well as the prognosis at 90 days.