Objective It is still unclear to predict the postoperative drainage volume of chronic subdural hematoma (CSDH) and the relation between the degree of brain shifting and the postoperative drainage volume, even CSDH is relatively common disease. In this study, the authors evaluate the relationship between the brain shifting index and the postoperative drainage volume, and investigated the influencing factors associated with the postoperative drainage volume in CSDH. Methods Fifty-eight CSDH with 45 patients, who were operated from November 2003 to April 2006, were enrolled. We investigated multiple factors associated with CSDH including the hemorrhage density and measured the parameters about the brain compliance on computed tomography (CT) as it followed (A, the distance between both inner skull table at the level of frontal horn of lateral ventricle; B, the distance between the most medial side of caudate nucleus head and the ipsilateral inner skull table at the same level of the parameter A; C, the distance of midline shifting at the level of third ventricle; D, the largest thickness of chronic subdural hematoma). We estimated the brain shifting index using the degree of frontal horn compression and midline shifting by the B/A or C/D ratios and the statistical analysis for the relation with the postoperative drainage volume was done. Results The mean age was 63.8±25.2 years and mean drainage volume was about 248.8±173.2 mL. Mean drainage date was 5.9±1.5 days and the daily drainage volumes were 109.5, 50.9, 22.2, 14.4, 15.8 mL respectively. The postoperative drainage volume of CSDH was significant correlated with the density of CSDH in brain CT (p=0.04), and the postoperative drainage volume was decreased as the B/A ratio increased (r=0.299, p=0.023). Conclusion The postoperative drainage volume in CSDH was affected by brain expansion. The density types of CSDH and the brain shifting index which were presenting the degree of frontal horn compression may helpful to predict the postoperative drainage volume in CSDH.