Objective：To investigate the effect of sufentanil combined with tramadol（for patient-controlled intravenous analgesia，PCIA） on postoperative analgesia and stress response in preschool children with choledochal cyst. Methods：A total of 40 children undergoing an elective total choledochal cyst excision were enrolled and randomized into PCIA group（group P） and control group（group C）（n=20）. The group P was given a formulation of 2 ?滋g/kg sufentanil，5 mg/kg tramadol，5 mg dexamethasone，and 4 mg ondansetron，diluted to 100 mL by normal saline；the group C was not given any postoperative analgesics. The Wong-Baker scores and adverse reactions of the children were collected at 1 hour before surgery（T1） and 12 hours（T2），24 hours（T3），and 48 hours after surgery（T4）；meanwhile，peripheral venous blood samples were collected from the children at the above four time points for testing of plasma levels of epinephrine（E），norepinephrine（NE），and 5-hydroxytryptamine（5-HT） by ELISA. Results：There was no significant difference between the two groups in the general condition of the children. A repeated-measure analysis of variance showed that there were significant differences in the Wong-Baker score between different groups and at different time points，with a significant group × time interaction effect observed（P<0.05）；there were significant differences in the plasma levels of NE and 5-HT between different groups and at different time points（P<0.05），with an increasing trend followed by a decrease observed in the plasma levels of E，NE，and 5-HT. Compared with the group C，the group P had significantly lower Wong-Baker scores（5.30±1.63 vs. 3.90±1.65，4.50±1.10 vs. 3.10±1.37，2.40±1.23 vs. 1.50±1.28；all P<0.05） and plasma levels of NE（574.71±100.95 vs. 514.13±75.32 pg/mL，483.56±53.47 vs. 441.32±69.53 pg/mL，404.61±59.36 vs. 364.83±49.33 pg/mL；all P<0.05） and 5-HT（290.81±47.31 vs. 262.35±30.31 pg/mL，248.40±48.52 vs. 209.38±39.02 pg/mL，232.16±33.75 vs. 199.48±30.94 pg/mL；all P<0.05） at T2，T3，and T4，respectively. There were no significant differences between the two groups in the number of analgesia-related adverse reactions，incidence of short-term postoperative complications，and length of hospital stay after surgery（P>0.05），but the unplanned extubation rate was significantly lower in the group P than in the group C（P<0.05）. The plasma levels of NE and 5-HT were positively correlated with the Wong-Baker score，with correlation coefficients of 0.38 and 0.56，respectively（P<0.05）. Conclusion：PCIA using sufentanil combined with tramadol is safe for postoperative analgesia in preschool children with choledochal cyst. It can effectively relieve postoperative pain，promote postoperative recovery，and reduce postoperative stress response in children without obvious adverse reactions.