Objective: To investigate the correlation between the level of serum 25-hydroxyvitamin D [25(OH)D] and infarction volume in patients with acute ischemic stroke (AIS) with internal carotid artery system (anterior circulation)., Methods: A prospective cohort study was conducted. Patients with AIS admitted to the department of emergency of Beijing Boai Hospital from October 2017 to September 2019 were enrolled. Nutritional risk screening 2002 (NRS 2002) were assessed in all cases within 24 hours after enrollment. Fasting venous blood was collected for biochemical analysis, including albumin (ALB), homocysteine (HCY), uric acid (UA), hypersensitive C-reactive protein (hs-CRP), etc. Serum 25(OH)D level was detected by electrochemiluminescence immunoassay. Magnetic resonance imaging (MRI) was performed to calculate the volume of cerebral infarction. According to the volume of cerebral infarction, the patients were divided into small volume (≤ 1 cm 3 ) group, medium volume (1 cm 3 < infarct volume < 20 cm 3 ) group and large volume (≥ 20 cm 3 ) group. The differences of serum 25(OH)D and other indicators in each group were compared; the influencing factors of infarct volume were analyzed by Logistic regression; and the goodness of fit of regression model was tested by Hosmer-Lemeshow (HL)., Results: A total of 224 patients with AIS were enrolled, 92 in small volume group, 90 in medium volume group and 42 in large volume group, and there was no significant difference in serum 25(OH)D level among small, medium and large volume groups [μg/L: 13.21 (7.47, 19.33), 11.20 (7.00, 15.07), 9.19 (6.30, 17.10), H = 4.994, P = 0.082]. There were 124 patients with AIS in anterior circulation, 45, 56 and 23 patients in the small, medium and large volume groups, respectively, with the increase of the cerebral infarction volume, the serum 25(OH)D level in small, medium and large volume groups decreased gradually, and the difference was statistically significant [μg/L: 13.22 (9.00, 19.65), 10.41 (6.72, 14.92), 8.30 (4.70, 11.30), H = 11.068, P = 0.004]. In addition, with the increase of the cerebral infarction volume, the older the patients with AIS in anterior circulation [years old: 63.0 (54.0, 75.5), 76.0 (63.0, 84.0), 82.0 (67.5, 85.0), H = 14.981, P = 0.001], the higher the nutritional risk ratio (35.6%, 53.6%, 73.9%, χ 2 = 9.271, P = 0.010), the higher the serum hs-CRP level [mg/L: 1.91 (0.92, 3.40), 4.10 (1.73, 22.42), 19.74 (4.02, 68.81), H = 21.477, P < 0.001], and the lower the ALB level (g/L: 42.30±12, 38.11±5.06, 35.14±5.49, F = 19.347, P < 0.001). After adjusting for age, gender, atrial fibrillation, nutritional risk, hs-CRP and other confounding factors, serum 25(OH)D was an independent protective factor for the infarct volume of AIS in anterior circulation [odds ratio (OR) = 0.962, P = 0.040], For every 10 μg/L decrease of 25(OH)D, the risk of one grade increase in infarction volume was increased by 47.7% respectively (goodness of fit: χ 2 = 5.357, P = 0.719)., Conclusions: The low serum 25(OH)D level was associated with the increase of infarct volume in the anterior circulation cerebral infarction, and early detection of serum 25(OH)D level can help risk stratification of AIS patients.