Objective? To investigate the change of 25 hydroxy-vitamin D[25（OH）D] concentration in children with respiratory tract infection of different sex and age，and analyze the correlation between 25（OH）D and immunoglobulin. Methods? A total of 280 children with respiratory tract infection in the Mianyang Central Hospital from June 2019 to February 2020 were included and divided into neonatal group，infant group，infant group and preschool group by age. The concentration of 25（OH）D was divided into 25（OH）D deficiency，25（OH）D，25（OH）D adequacy and 25（OH）D excess. High performance liquid chromatography tandem. Concentration of serum 25（OH）D was determined by mass spectrometry and serum immunoglobulin was determined by scattering turbidimetric immunoassay. Results? There was significant difference in the concentration of 25（OH）D in children with respiratory tract infection at different ages，and the composition of 25（OH）D excess，adequacy，deficiency and deficiency in children of different age groups was different，the difference was statistically significant（P＜0.01）. The concentration of 25（OH）D in children with respiratory tract infection of different sex was statistically significant（P＜0.01），and the composition ratio of 25（OH）D concentration in children with respiratory tract infection was significantly different. The concentration of immunoglobulin was significantly different. The concentration of 25（OH）D was correlated with IgA，IgM and IgE（r=0.424，0.496 and 0.465，P=0.000，0.000 and 0.000，respectively）. Conclusion? 25（OH）D in children with respiratory tract infection is associated with immunoglobulin. Vitamin supplements for children with respiratory infections D help improve immune function，enhance immunity.