Abstract: Abstract Objective：To investigate the protective effect of ambroxol on lung injury in children during cardiopulmonary bypass. Methods：Forty patients aged 4-9, scheduled operation of atrial septal defect (asd)\ventricular septal defect (vsd), were randomly allocated into two groups, ambroxol group (group M, n=20) and control group (group C, n=20). Ambroxol (4.5mg/kg in 50 ml of saline) was injected before cardiopulmonary bypass (CPB) in group M. The equal volume of normal saline was infused in group C. Blood samples were taken to detect white blood cells (WBC) before operation, after anesthesia (T1), 10 min after administration (T2), 5 min after CPB (T3), 1 h after ICU admission (T4), 2 h after ICU admission (T5), before decannulation (T6). Results：There were significant differences in R factors and time effect between two groups (P<0.001), and there were significant differences in T5 and T6 between two groups (P<0.01), which reached the highest level in T4 in two groups and then showed a downward trend. There was no significant difference in lung compliance (CL) in T1 and T5 between two groups (P >0.05). There were significant differences in values of WBC between two groups (P<0.001). There was a significant interaction between processing factors and time effects (P<0.01). There were significant differences in the peak inspiratory pressure (pip) and plateau pressure (Plat) between two groups (P<0.01), which showed a significant interaction between processing and time effects (P<0.01). Conclusion：Ambroxol has the protective effect on lung injury induced by CPB.