Objective To study the relationship between vitamin D deficiency and severe bronchial pneumonia in children. Methods A total of 58 children with severe pneumonia complicated by a lack of vi-tamin D from January 2011 to December 2013 were enrolled in this study,the serum 25-( OH) D3 was detec-ted by the high performance liquid chromatography-tandem mass spectrometry. All the children were random-ly divided into two groups,the vitamin D treatment group( observation group) and without vitamin D treat-ment group(control group),control group received conventional treatment,and the observation group re-ceived vitamin D based on the same treatment with the control group. Results The serum 25-( OH) D3 of the observation group increased significantly( P<0. 01 ) . ( 2 ) The symptoms such as cough, asthma, fever and pulmonary rales in the observation group disappeared more quickly than that of the control group,and the av-erage cure days of heart failure,respiratory failure and encephalopathy as well as the average length of stay in the observation group were less than those of control group,and there were significant differences between two groups(P<0. 01). (3) The total efficiency of the observation group and control group was 96. 42% and 73. 33% respectively,and there was significant difference between two groups(χ2 =6. 693,P<0. 01). (4) Blood oxygen pressure and oxygen saturation of the observer group were significantly improved,CO2 partial pressure decreased obviously, the differences were statistically significant between two groups ( P <0. 01 ) . Conclusion During the treatment periods in children with severe pneumonia,vitamin D supplements could relieve symptoms promptly,shorten the duration,improve the 25-( OH) D3 levels,and vitamin D deficiency may also be the underlying cause of severe pneumonia in children.