The more accepted theory of the pathogenesis of subcerebellar tonsillar herniation malformations is that dysplasia of the occipital bone results in a congenitally small volume of the posterior cranial fossa, and that overcrowding of the normally-developed hindbrain portion of the brain allows the subcerebellar tonsillar portion of the cerebellar tonsils to herniate into the vertebral canal. Although multiple theories exist to explain the pathogenesis of spinal cord cavernous effusion, they all center on the following: herniation of the inferior cerebellar tonsils leads to obstruction of cerebrospinal fluid circulation in the cranial cavity and spinal canal, which, in turn, causes enlargement and accumulation of fluid in the central canal of the spinal cord. Surgery is the only effective treatment for Chiari I malformation combined with hydrocele in the spinal cord cavity. The key steps in surgery are complete release of the stenosis in the region of the occipital foramen magnum and proper enlargement of the volume of the posterior cranial fossa. In recent years, smaller bone windows (even 1.5 cm × 2 cm) have been shown to yield good decompression results, making it possible to treat Chiari I malformation with hydrocele using minimally invasive techniques. The Neurosurgery Spinal Cord and Spine Specialty Group applied neuroendoscopic illumination, observation of the operative field, and endoscopic extrasheath operation to perform atlanto-occipital decompression surgery for the treatment of Chiari I malformation with spinal cord cavernous effusion, and the surgical results were satisfactory. All patients had a smooth postoperative recovery without complications such as central nervous system infection and occipital subcutaneous effusion, and there was no case of surgical death. The postoperative hospitalization time ranged from 3 to 14 days, with an average hospitalization time of 6 days; the hospitalization cost was around 35,000 RMB. The patients were followed up after surgery, and all of them showed improvement. MRI was reviewed at the time of follow-up, and the spinal cord cavities of most patients were reduced to different degrees compared with the previous ones.