Protection roles of serum vitamin D levels in the cardiovascular system. This observational study enrolled 286 young ACI patients with type 2 diabetes mellitus in our Hospital from October 2016 to April 2020 as the study group. 127 healthy individuals as the control group. Serum levels of vitamin D and blood pressure indexes of the two groups were measured and the correlation between vitamin D levels and the clinical phenotype of young cerebral infarction was explored. Significantly higher 25-hydroxyvitamin D was observed in the study group vs. control group. During the day, The values of systolic and diastolic blood pressure and mean blood pressure was well-balanced between the two groups. However, at night and within 24 hours, significant differences in systolic and diastolic blood pressure and mean value were observed between the two groups. The higher the vitamin D level, the greater the decrease in blood pressure at night. Serum 25-hydroxyvitamin D was positively correlated with the reduction of night systolic and diastolic blood pressure. Low levels of 25-hydroxyvitamin D led to a small decrease in blood pressure at night, resulting in a vulnerable cardiovascular system and an increased incidence of cerebral infarction in the young population.