Limited enlargement of the posterior cranial fossa is used for the narrow posterior cranial fossa. In this type of patients, the bone of the occipital crest and the posterior border of the foramen magnum are abnormally thickened and invaginated. Therefore, the posterior margin of the foramen magnum should be removed as widely as possible during the enlargement of the posterior cranial fossa, and the occipital bones should be removed up to the inferior collar line. The posterior atlantoaxial arch and the inferior herniated tonsils can be removed according to the situation and the dura mater can be enlarged and repaired. In the atlanto-occipital region with Chiari malformation, a transoral pharyngeal approach should be chosen for dentate grinding. The foramen magnum area is enlarged by the removal of the dentate, the medulla oblongata is decompressed, the cerebellar tonsils are retracted, and the spinal cord cavity (fluid retention) is reduced. If stage II surgery is required to perform occipito-cervical implant fusion fixation, the occipital bone will not be immobilized due to decompression. Posterior decompression in such patients is bound to aggravate the condition and create a significant obstacle to subsequent repair surgery. Adverse results of certain procedures: extensive posterior cranial fossa enlargement, simple posterior cranial fossa decompression, and cavity shunting have been used in the past. Patients return to the clinic after surgery, mostly because of aggravation of symptoms or other symptoms, mainly: 1. Excessive bony occlusion of the posterior cranial fossa, loss of cerebellar support, cerebellar drop resulting in pseudocerebellar bulge, and widening of the pontine and superior cerebellar pools. Such patients are often seen in the outpatient clinic with no change in the cavity and symptoms of cerebral nerve provocation and cerebellar symptoms. There is diplopia due to stretching of the spreading nerve, tinnitus, nystagmus and vertigo due to stretching of the cochlear vestibular nerve, facial palsy due to stretching of the trigeminal nerve, and limited neck movement due to excessive stripping of the occipital muscles and loss of attachment points. It is possible to improve the surgery again, but it is much more difficult to operate. 2. The effect of “posterior cranial fossa decompression” without cutting the dura is very limited by occluding the occipital bone. Such patients are often seen in outpatient clinics, with no obvious signs of “enlargement” of the posterior cranial fossa and no improvement of clinical symptoms. If the “occipital pool reconstruction” is performed again, it is found that the dura is often partially ossified, which continues to affect the volume of the posterior cranial fossa. Although the cavity of the spinal cord can be reduced in the early stage by simple spinal cord cavity shunt, the compression of the medulla oblongata by the cerebellar tonsils does not improve, and the clinical symptoms of spinal nerve irritation at the shunt are all present. In some patients, late MRI examination revealed no change in the cavity, requiring reoperation.