Trephine syndrome and post-traumatic hydrocephalus are frequent complications following decompressive craniectomy in patients with traumatic brain injury. The recommended treatments for trephine syndrome and post-traumatic hydrocephalus include cranioplasty and ventriculoperitoneal shunt placement. However, There is no consensus on the priority of these two procedures. We describe the case of a 17-year-old adolescent who sustained a severe head injury in a traffic accident and underwent a right frontotemporal parietal craniectomy and contusionectomy to relieve brain herniation. The patient subsequently developed severe brain tissue collapse combined with low cranial pressure hydrocephalus. Given that prior cranioplasty could alleviate the direct pressure of external atmospheric pressure on the brain and enhance cerebrospinal fluid circulation, we prioritized cranioplasty for the patient. Following cranioplasty, the patient's cerebrospinal fluid pressure increased from 80 mmHg to 95 mmHg, indicating a slight elevation compared to the preoperative period. Additionally, the imaging revealed that the patient's brain tissue was no longer severely collapsed. From this case study, we believe that the criteria for surgical sequence in patients with severe flap collapse associated with hydrocephalus should be further research, and such patients should be identified early and intervened early should be performed through outpatient follow-up in clinical practice.