Objective To investigate the risk factors for subdural hygroma (SDG) after decompressive craniectomy (DC) for traumatic brain injury.Methods A retrospective analysis was performed on the clinical data and imaging data of 138 patients who underwent DC for traumatic brain injury in Department of Neurosurgery in our hospital from January 2014 to June 2016. According to the presence or absence of SDG after DC, the patients were divided into SDG group and non-SDG group. The univariate t test, chi-square test, and multivariate logistic regression analysis were used to identify the risk factors for SDG after DC.Results Of the 138 patients, 85 (61.6%) had SDG. The univariate analysis showed that there were no significant differences in sex, age, preoperative Glasgow Coma Scale score, compression of the cisterna ambiens, incidence of cerebral hernia before surgery, epidural hematoma, intracerebral hematoma, intraventricular hemorrhage, subarachnoid hemorrhage, anteroposterior diameter of the bone flap, greatest diameter of the bone flap, and side of DC between the SDG group and the non-SDG group (P>0.05). SDG after DC was significantly associated with midline shift >5 mm, cortical opening, subdural hematoma, and distance between the margin and midline of the bone flap <2 cm (P<0.05). The multivariate analysis showed that midline shift >5 mm and cortical opening were independent risk factors for SDG after DC (P<0.05).Conclusions For patients treated with DC for traumatic brain injury, SDG is significantly associated with midline shift >5 mm and cortical opening. The potential progression into SDG should be taken seriously.