Ｏｂｊｅｃｔｉｖｅ　To investigate the relationship between blood lactic acid(Lac) and cooling rate during the pre-parallel bypass period in infants undergoing cardiac surgery.　Methods　Forty four infants with congenital disease, no limit on the gender, no more than 5 years old. After anesthesia induction and clamping the ascending aorta, respectively collecting 2 ml of radial artery blood to conduct blood gas analysis. Measuring the Lac level, oxygen partial pressure(PaO2), Hb, and also recording MAP, body surface area, the amount of colloidal liquid and crystal liquid in priming fluid, nasopharyngeal temperature when clamping the ascending aorta, the time from cooling to clampingthe ascending aorta. Calculating the change of Lac level and cooling rate. Considering the balance between groups, divided the patients into two groups according to the change of Lac level, group A(≥0.5 mmol/L) and group B(＜0.5 mmol/L)(n=22).　Results　According to the results of t-test, Lac value of both groups are increased after anesthesia induction and a significant rise in group A than in group B， group A was (0.8±0.3) mmol/L vs (1.8±0.4) mmol/L(P<0.05), group B was(0.9±0.4) mmol/L vs (1.4±0.9) mmol/L(P<0.05), while the cooling rate of group A is slower than that in group B[(0.9±0.2)　℃/min vs (0.5±0.2)　℃/min](P<0.05). Other factors have no statistical difference between the two groups.　Conclusions　The pre-parallel bypass period of extracorporeal circulation, to the extent permitted temperature difference between blood and water, increasing the cooling rate can inhibit the rise of lactic acid level in this stage.