The causes of chronic subdural effusion are as follows: First, brain atrophy, when the brain atrophy of the elderly is more obvious, the retraction of brain tissue leads to the increase of subdural space, and the cerebrospinal fluid will slowly penetrate into the subdural and fill up the subdural space to form subdural effusion. Secondly, head trauma is also the most common cause of subdural effusion. Head trauma will lead to arachnoid membrane rupture in the skull, and the cerebrospinal fluid will enter into the subdural cavity through the arachnoid membrane rupture, resulting in subdural effusion. Third, chronic subdural hematoma will evolve into subdural effusion after surgery. After subdural hematoma removal surgery, some patients will have difficulty in brain tissue reset, and the subdural cavity will exist for a long time, resulting in the occurrence of effusion. Fourth, medical factors, after craniotomy surgery caused damage to the arachnoid membrane, postoperative cerebrospinal fluid will enter the subdural space of the brain through the arachnoid fissure, forming subdural effusion.