Objective To discuss the risk factors for recurrent respiratory tract infection (RRTI) in preschool children,and to analyze its correlation with vitamin D (VitD) level,in order to provide reference for preventing RRTI in preschool children. Methods Totally 98 cases children aged 2 to 6 years with RRTI in Shaoxing Central Hospital from August 2015 to August 2017 were selected as the observation group,and 101 healthy children admitted in the same period were selected as control group.Data on maternal pregnancy,preterm birth,birth weight,breastfeeding time,outdoor activities,second-hand smoking and the history of allergic diseases were collected by questionnaire survey.The serum levels of vitamin D,IgA,IgG,IgM,CD4+,CD8+,CD4+/CD8+and other humoral immune indexes in observation group and control group were determined.For children with VitD deficiency (<20 ng/ml),these indexes were tested again in three months after VitD supplementation.Besides,children with VitD deficiency were followed up to 12 months,and the times for respiratory tract infections in children were counted. Results Preterm birth,low birth weight,breastfeeding time,living environment,outdoor activities,VitD,IgA,CD4+,CD4+/CD8+ levels were the risk factors of RRTI.The serum levels of IgA,CD4+,CD4+/CD8+ were higher in children after 3 months of VitD supplementation,and the times for infection in one year was significantly lower than that before VitD supplementation (P<0.05).There were positive correlations among vitamin D level and IgA,CD4+,CD4+/CD8+ before and after treatment (r=0.56,0.73,0.68,P<0.05).There was a negative correlation between VitD level and the times for respiratory tract infection (r=-0.64,P<0.05). Conclusions Preterm birth,low weight,breastfeeding time,living environment and outdoor activities are risk factors for RRTI in children.VitD level and immunological indexes are abnormal in children with RRTI.Increasing VitD level can improve the immunity of children with RRTI and reduce the recurrence of respiratory tract infection.