Objective There are currently limited studies on the hemodynamics of posterior cerebral artery （PCA） compensation in patients with severe stenosis or occlusion of the internal carotid artery （ICA）， and this study aims to investigate the application value of transcranial Doppler （TCD） in assessing PCA compensation after severe stenosis or occlusion of the unilateral extracranial ICA. Methods A retrospective analysis was performed for 107 patients who were hospitalized in Encephalopathy Center， Guangdong Provincial Hospital of Traditional Chinese Medicine， from January 2022 to February 2025 and met the diagnostic criteria for severe stenosis or occlusion of the unilateral extracranial ICA， and all patients underwent both digital subtraction angiography （DSA） and TCD. The patients were divided into groups based on PCA compensation on DSA， and TCD quantitative parameters were analyzed. The receiver operating characteristic （ROC） curve was plotted to investigate the value of TCD in assessing graded compensation of the PCA. Results Among the 107 patients， there were 75 patients （70.1%） in the compensation group and 32 patients （29.9%） in the non-compensation group. The optimal cut-off value of TCD in assessing PCA compensation was a mean velocity of ≥65 cm/s at the PCA P1 segment， with an area under the ROC curve（AUC） of 0.869（95% CI 0.785‒0.953）， a sensitivity of 78.7%， a specificity of 84.4%， and a Youden index of 0.630. Compared with the non-compensation group， the compensation group had significant increases in peak systolic velocity， end-diastolic velocity， and mean velocity （P<0.01） and a significant reduction in pulsatility index （P<0.05）. As for the graded compensation of the PCA， the patients with posterior communicating artery （PCoA） compensation alone accounted for 57.3% （43/75）， those with leptomeningeal collateral compensation of the PCA alone accounted for 16.0%（12/75）， and those with both patterns of compensation accounted for 36.7% （20/75）.An end-diastolic velocity of ≥48 cm/s at the PCA P1 segment was the optimal cut-off value for assessing PCoA compensation， with an AUC of 0.813 （95%CI 0.726‒0.900）， a sensitivity of 76.2%， a specificity of 79.5%， and a Youden index of 0.557； a mean velocity of ≥60 cm/s at the PCA P2 segment was the optimal cut-off value for assessing leptomeningeal collateral compensation of the PCA， with an AUC of 0.824 （95%CI 0.723‒0.925）， a sensitivity of 71.5%， a specificity of 90.7%， and a Youden index of 0.625. Conclusion This study conducts a hemodynamic stratification analysis on compensation patterns of the PCA in patients with severe stenosis or occlusion of the unilateral extracranial ICA. Quantitative blood flow indicators at the PCA-P1 and PCA-P2 segments are highly consistent with DSA in assessing PCA compensation and can reliably evaluate the compensation status of the PCoA and the leptomeningeal collaterals.