Objective：To investigate the roles of tidal breath flow-volume loop（TBFVL） in monitoring lung function of preterm infants with different degrees’ bronchopulmonary dysplasia（BPD）. Methods：One hundred and thirteen preterm infants without BPD and heart & lung disease-free were erolled as controls and 106 BPD infants were taken as observation group.Observation group was divided into mile，moderate and severe groups according to the definition of BPD. Parameters of lung function test，including TBFVL，functional residual capacity（FRC） and lung clearance index（LCI） were collected and analyzed when the patients were at 44 weeks postmen－strural age（PMA）. Results：①Shape of TBFVL showed that in observation groups：ratio of volume to peak tidal expiratory volume a－gainst expiratory volume（Vpef/Ve） was decreased，peak expiratory flow（TPEF） was markedly higher than peak inspiratory flow（TPIF）.Expiratory descending branch was descended sharply；expiratory descending branch of mild observation group was slightly gradient than that of control group；expiratory descending branch of moderate and severe groups was depressed to volume axis. ②Compared with parameters in control group：except moderate observation group，Vpef/Ve in mild and severe observation group were lower（P<0.05）；TPIF，TPEF，tidal expiratory flow at 75% remaining tidal volume（TEF75） in severe observation group were decreased compared with those in mild observation group and control group（P<0.05）；tidal expi－ratory flow at 25% remaining tidal volume（TEF25） was lower in mild and moderate observation groups than in control group，while it was higher in severe group than in control group（P<0.05）；FRC was lower in moderate and severe observation groups than in control group；only in moderate observation group；LCI was higher than moderate observation group in control group（P<0.05）.③Spearman correlation analysis showed that FRC，Vpef/Ve，TPIF，TPEF，TEF75，TEF25 had strong correlationship with the degree of BPD with statistical significances while LCI not. Conclusions：Because of the dysplasia of pulmonary alveoli and terminal small air－way，expiratory descending branch of TBFVL in BPD infants is descended sharply and in moderate and severe BPD infants even de－pressed to volume axis.TBFVL can directly reflect the lung function condition of BPD infants and can be used in clinical for lung de－velopement follow-up combining with FRC，LCI.