Inclusion Criteria: Participants enrolled spontaneously breathing preterm infants born between 24 and 29.9 weeks' gestational age with signs of respiratory distress syndrome.Preterm infants were receiving nasal continuous positive airway pressure(NCPAP) or non-invasive positive pressure ventilation (NIPPV) without intubation before surfactant administration. Patients diagnosed with NRDS on non-invasive respiratory support (NIPPV or NCPAP) were given a therapeutic dose of pulmonary surfactant via MISA within 120 minutes after birth. Parental consent was obtained for all participants before birth.  Exclusion Criteria:(1) Preterm infants who had been intubated prior to pulmonary surfactant administration due to postnatal resuscitation or other reasons; (2) preterm infants with obvious malformations; (3)Newborns who died or were transferred to other hospitals for surgery or whose information was incomplete;(4)Infants who participated in other interventional research projects.