Objective:To evaluate the accuracy of variation of carotid artery hemodynamic parameters combined with passive leg raising (PLR) test in predicting supine hypotension syndrome (SHS) after spinal anesthesia in the patients undergoing cesarean section.Methods:Sixty-four parturients who were at full term with a singleton fetus, at 37-42 weeks of gestation, aged 18-40 yr, with body mass index of 18-30 kg/m 2, of American Society of Anesthesiologists physical status Ⅰ or Ⅱ, undergoing elective cesarean section, were enrolled in this study.The variation of carotid artery diameter (ΔD), variation of velocity time integral (ΔVTI), and variation of carotid blood flow (ΔCBF) before and after PLR were measured using ultrasound.Patients were divided into SHS group and non-SHS group (NSHS group) according to whether SHS after spinal anesthesia occurred.Pearson correlation was used to analyze the correlation between ΔD, ΔVTI, ΔCBF and systolic blood pressure (SBP) after spinal anesthesia.The receiver operating characteristic curve was used to assess the accuracy of ΔD, ΔVTI and ΔCBF in predicting SHS. Results:ΔVTI was negatively correlated with SBP after spinal anesthesia ( r=-0.539, P<0.01), ΔCBF was negatively correlated with SBP after spinal anesthesia ( r=-0.475, P<0.05), and ΔD had no correlation with SBP after spinal anesthesia in group SHS ( P>0.05). The critical values of ΔCBF, ΔVTI, and ΔD combined with PLR in predicting SHS after spinal anesthesia were 15.5%, 10.1%, and 6.0%, respectively, the sensitivity was 92.9%, 57.1%, and 96.4%, respectively, and the specificity was 53.1%, 81.2%, and 75.0%, respectively, and the areas under the curve were 0.873, 0.681 and 0.846, respectively. Conclusion:The ultrasound-measured ΔCBF and ΔD of carotid artery combined with PLR can be used as a reliable method to predict SHS after spinal anesthesia in the patients undergoing cesarean section, and the ΔCBF combined with PLR has a higher accuracy.