Objective：To investigate the association between vitamin D（VitD）deficiency and the incidence and severity of bronchiolitis in infants and young children，and the relationship between VitD level and response of alveolar epithelial cells in vitro. Methods：A total of 80 children with bronchiolitis diagnosed during the period from January 2016 to December 2017 in Jiangning Hospital Affiliated to NMU were selected. Severity scores were given according to the standard. Forty healthy infants were selected as the control group. The level of 25-（OH）D was explored in relation to the severity score of bronchiolitis，and the levels of histamine and TNF-α in the two groups of children were examined to investigate the relationship between the level of VitD and in vivo VitD levels. Human alveolar epithelial cells A549 were cultured in groups A，B，and C. Group A served as the control group and were cultured in the original culture medium. Group B/C were infected with respiratory syncytial virus（RSV）for 24 hours. Group C was treated with calcitriol at the same time for 24 h. The levels of histamine and TNF-α in the culture supernatant and the differences of VitD receptor（VDR）and CYP27B1 mRNA expression in the cells of three groups were measured separately after 24 h. Results：The 25-（OH）D level in the bronchiolitis group was significantly lower than that in the healthy control group（P < 0.001）. The severity score of bronchiolitis was negatively correlated with the 25-（OH）D level in vivo（R2=0.5398）. The levels of serum histamine（P < 0.01）and TNF-α（P < 0.05）in the bronchiolitis group were significantly higher than those in the control group. After RSV infected cells in vitro，the levels of histamine and TNF-α in the culture supernatant of group B increased significantly（both P < 0.001）. The expression of VDR and CYP27B1 mRNA in group C increased significantly（both P < 0.001）. Conclusion：Increasing the VitD level in infants and young children can reduce the incidence and severity of bronchiolitis，which may be related to inhibiting inflammatory mediators released by type Ⅱ alveolar epithelial cells and promoting VitD activation and utilization.