Abstract： Objective:To explore the effects of different levels of colloid osmotic pressure (COP) on postoperative recovery of infants during cardiopulmonary bypass (CPB).Methods:From December 2019 to July 2020, the relevant clinical data were retrospectively reviewed for 67 children with congenital heart diseases (CHD) undergoing initial corrective surgery under CPB. There were 29 boys and 38 girls with body weight <8 kg. They were assigned into two groups of low COP (n=34) and high COP (n=33). Postoperative coagulation function, hepatorenal function, postoperative fluid volume, delayed chest closure rate, mechanical ventilation time, intensive care unit (ICU) duration and hospitalization stay were recorded.Results:No significant inter-group difference existed in COP value at pre-CPB ( P>0.05) and COP values dropped to the lowest at 10 min after transition and then rose slowly. Significant differences existed in COP at 10 min after CPB, myocardial protective fluid injection and stop time ( P<0.05). At 24 h, amount of albumin used was lower in low COP group than that in high COP group [(3.18±4.29) vs (6.47±6.09) g, P<0.05]. Lactate value of low COP group was higher than that of high COP group at 72 h postoperatively [(2.25±3.19) vs (1.16±0.66) mmol/L, P=0.05]. At 24h, fluid requirement was higher in low COP group than that in high COP group [(558.18±220.23) vs (442.00±156.25) mL, P<0.05). At 24h, fluid balance of low COP group was better than that of high COP group [(-73.36±170.26) vs (-26.43±298.12) mL, P<0.05]. No significant differences existed in hospitalization stay, mechanical ventilation time, ICU duration, rate of re-operation, rate of delayed chest closure, rate of ventilator use >7 days, rate of peritoneal dialysis or rate of renal replacement therapy ( P>0.05). At 24h, creatinine value of low COP group was lower than that of high COP group [(30.64±8.98) vs (40.54±25.24) μmol/L]. At 72h, creatinine value was lower in low COP group than that in high COP group [(26.14±8.95) vs (39.85± 27.52) μmmol/L, P<0.05). Conclusion:Higher colloid osmotic pressure level is beneficial for maintaining a decent fluid balance, reducing postoperative fluid input and lowering postoperative blood lactic acid level and mechanical ventilation time. Yet it has little effect on liver function and coagulation function.