Objective：To discuss the effects CO2 pneumoperitoneum on acid-base balance in children after laparoscopic pyeloplasty and its prevention measures. Methods：Totally 64 patients underwent laparoscopic pyeloplasty and 64 patients underwent open Ander－son-Hynes pyeloplasty from January 2010 to June 2011 were divided into laparoscopic group and open group. All patients were divid－ed into infants group，preschool-age group and school-age group respectively. Twenty-six patients underwent laparoscopic pyeloplasty from July 2011 to February 2012 were selected as laparoscopic air replacement group，in which peritoneal residual CO2 was sufficiently replaced with air at the end of operation. Blood gas analysis was made at 4 h after operation for all patients. Results：Hypercapnia incidence of laparoscopic group was 35.9%，higher than that of open group（18.7%）（?字2=4.578，P=0.029）. Incidences of acid-base imbalance in infants group and preschool-age group were 91.3%，84.2%（laparoscopic group） and 84.4%，76.5%（open group） respective－ly，higher than those in school-age group（50.0%，33.3%）（P<0.05）. Hypercapnia incidence of laparoscopic air replacement group was 11.5%，lower than that in laparoscopic group（35.9%）（P<0.05）. Conclusions：Peritoneal residual CO2 in laparoscopic pyeloplasty is the major influence factor that can cause postoperative hypercapnia in children. The younger the age，the greater the effect of CO2 pneu－moperitoneum on the acid-base balance. Sufficiently replacing peritoneal residual CO2 is the important measure that can decrease postoperative hypercapnia incidences.