Objective To investigate the relationship between serum vitamin D level and acute ischemic stroke caused by large artery atherosclerosis (LAA) and the effects of serum vitamin D on its severity and recent neurological function improvement. Methods Totals of 100 acute ischemic stroke patients with LAA within 24 h hospitalized in Department of Neurology， the Fourth Hospital of Baotou were included in test group. NIHSS scores were evaluated before and after 14 days of treatment. 60 healthy controls were enrolled in control group. The serum levels of vitamin D in two groups were measured and compared. Patients were divided into 3 groups according to NIHSS scores: the mild group， moderate group and severe group. And the differences of the serum vitamin D in observation group and control group were compared. According to the level of serum vitamin D， patients were further divided into 3 groups again: the sufficient group， insufficient group and deficiency group. The relationships between serum vitamin D and severity and recent neurological function improvement in patients with LAA were evaluated. Results The level of serum vitamin D in test group was significantly lower than that in control group［ (13.97±1.32)μg/L vs (21.62±0.89)μg/L；P=0.018］. The level of serum vitamin D in the severe group was significantly lower than that in moderate group and mild group［ (9.21±0.73)μg/L vs (12.24±1.02)μg/L， (15.01±0.98)μg/L；P＜ 0.01］. Serum vitamin D level was negatively correlated with NIHSS score (r=-1.892，P=0.046 ). After 14 days of treatment， ΔNIHSS in sufficient group was significantly higher than that in insufficient group and deficiency group［ (3.93±1.21)μg/L vs (2.96±1.83)μg/L， (2.05±2.06)μg/L； F=5.47，P=0.01］. Conclusions The level of the serum vitamin D is significantly lower in acute ischemic stroke patients caused by LAA and is closely related to the recent neurological function improvement.