Herein, we report an unusual case of a patient who presented with a severe, sinking skin flap after a decompressive craniectomy and ventriculoperitoneal shunt surgery due to a traumatic brain injury. After cranioplasty, the patient's neurological deficiency improved and was confirmed by transcranial Doppler sonography. In addition to discussing the pathogenesis of the sinking skin flap, we emphasize the importance of cranioplasty for neurological improvement and remind the surgeon to "never neglect the atmospheric pressure effect on a brain with a skull defect".