Ｏｂｊｅｃｔｉｖｅ　To evaluate the clinical application of preoperative entropy in the assessment of severity and prognosis of patients undergoing brain injury surgery.　Methods　A total of 61 patients undergoing brain injury surgery were enrolled. Entropy before operation was recorded under three conditions including no-stimulation, voice stimulation and pain stimulation. The patients were divided into two groups according to the Glasgow Coma Scale (GCS) before operation: a mild group (GCS 9-15, n=19) and a severe group (GCS≤8, n=42). Both groups were compared for their entropies under the three conditions. Additionally, the patients were divided into another two groups according to the Glasgow Outcome Score (GOS) three months after operation: a good prognosis group (GOS 4-5, n=34) and a poor prognosis group (GOS≤3, n=27). The two groups were compared for their entropies under the three conditions.　Results　All the patients were enrolled into the follow-up observation. There were 19 cases in the mild group and 42 cases in the severe group. Compared with the mild group, the severe group presented remarkable decreases in entropy under the three conditions (P<0.01). There were 34 cases in the good prognosis group and 27 cases in the poor prognosis group. Compared with the good prognosis group, the poor prognosis group presented remarkable decreases in entropy under the three conditions (P<0.01).　Conclusions　Preoperative entropy can be used as an indicator to assess the extent of brain injury and evaluate the prognosis of the patients undergoing acute brain injury.