Objective To investigate the development of tidal breathing lung function in very low birth weight infants with bronchopulmonary dysplasia(BDP).Methods Two hundred and sixty-two very low birth weight infants hospitalized in Wenzhou Children′s Hospital were enrolled as objects.All infants were given the tidal breathing pulmonary function test within a week before discharge and at the corrected age of 6 to 8 months.According to clinical diagnosis,they were divided into BPD group(n=65) and non-BPD group(n=197),and BPD group were divided into mild group(n=31),moderate group(n=20) and severe group(n=14) according to the severity.The lung function indexes were compared between different groups.Results The results of the test within a week before discharge displayed that the respiratory rates in the BPD groups were higher than those in the non-BPD group(all P＜0.05).The peak expiratory flow(TPEF),tidal expiratory flow at 75%,50%,and 25% tidal volume(TEF75,TEF50,TEF25) in moderate to severe BPD group were higher than those in other groups(all P＜0.05),while in mild BPD group were higher than those in non-BPD group(all P＜0.05).The ratio of time to peak tidal expiratory flow to total expiratory time (TPEF/TE),ratio of volume to peak tidal expiratory flow to total expiratory volume (VPEF/VE) were lower in BPD group than those in non-BPD group,the decline was more obvious as the BPD was more serious(all P＜0.05).No significant differences were found between groups in tidal volume(P＞0.05).The result at the corrected age of 6 to 8 months displayed that TPEF,TEF75,TEF50,TEF25 in severe BPD group were still higher than those in any other group(all P＜0.05),while TPEF/TE,VPEF/VE were lower than those in any other group(all P＜0.05),no significant differences were found between other groups in other indicators(P＞0.05).Conclusion Preterm infants BPD have different degrees of lung injury,but as the age grows some lung function indexes can improve gradually(at the corrected age of 6 to 8 months).However,early severe small airway obstruction is still serious.Therefore,it′s important to prevent and treat BPD for the prevention and regulation of respiratory tract disease in future.