To investigate the effect of continuous infusion of phenylephrine to prevent hypotension of women during cesarean section caused by combined spinalepidural anesthesia, and to analyze its dose-effect relationship. Methods: A total of 150 pregnant women with full-term who underwent elective cesarean section in hospital were selected as the research subjects between October 2018 and February 2021. These women were divided into 6 groups(25 cases in each group) by random number table method. All women had received intrathecal injection of 10mg specific gravity of bupivacaine and 5 μg of sufentanil for anesthesia. Then, the women in the group A, in group B, in group C, in group D, in group E, and in group F were given intravenous infusion of 2 μg, 4 μg, 6 μg, 8 μg, 10 μg, and 12 μg of phenylephrine diluted with normal saline to 5ml, respectively. The infusion of the women in the six groups was stopped at the delivery of baby. The heart rate, the blood pressure value, the maternal adverse reactions rate, and the results of fetal umbilical artery blood gas analysis of the women in the six groups were observed and recorded during anesthesia. The systolic blood pressure value of the women returning to the basic value and above was considered effective treatment of hypotension. Logistic regression analysis was performed to help draw the dose-effect relationship curve. The median effective dose(ED50) and 95% effective dose(ED95) were calculated. The first occurrence time of hypotension during the operation, the treatment of hypotension, and the postoperative complications situations of the women in the six groups were recorded. Results: There were no significant differences in the basal systolic blood pressure value, the time of first hypotension occurrence, the operation time, the urine volume after operation, and the time of hospital stay of the women among the six groups(P>0.05). There were no significant differences in the incidences of bradycardia, nausea and vomiting, and chills of the women among the six groups(P>0.05), but the effective rate of preventing hypotension of the women in group A, in group B, in group C, in group D, in group E, and in group F had increased gradually(P＜0.05). The analysis showed that the values of ED50(95%CI) and ED95(95%CI) of phenylephrine for preventing hypotension of the women caused by the combined epidural during anesthesia were 6.3 μg(5.0-7.5 μg) and 12.3 μg(10.1-13.7 μg), respectively. The value of HR, SBP, CI, SVI, or SVV of the women in the six groups when the placental separation was significantly higher than those before anesthesia, before delivery, or at the end of operation(P＜0.05), but which of the women had no significant differences among the six groups. There were no significant differences in the 1min Apgar score and 5min Apgar score of the newborns, and the values of buffuer excess(BE), lactic acid, and PH of the women among the six groups(P>0.05). Conclusion: The values of ED50(95%CI) and ED95(95%CI) of phenylephrine for preventing hypotension of the women caused by the combined epidural during anesthesia are 6.3 μg and 12.3 μg.