We report a case of a 61-year-old man with ankylosing spondylitis who showed cervical spine fracture. The patient had fallen down on the floor and presented with severe neck pain. He was treated conservatively with a hard neck collar in an emergency hospital because of C7 body fracture without dislocation. However, the follow-up radiographs demonstrated a progressive C6-7 anterior dislocation. He was referred to our hospital 6 weeks after the trauma. The 3D-CT reconstruction imaging revealed that the fracture extended from the C7 vertebral body to the C6 lamina via the bilateral C6/7 facet joints. The patient underwent C2-Th3 posterior fixation using pedicle and lateral mass screw techniques. The postoperative course was uneventful. He was discharged without any complication at 1 month postoperatively. The radiograph 3 months after surgery showed good bone fusion. Spine fracture with ankylosing spondylitis usually shows significant instability because of the long lever-arm of the fused vertebrae at the fracture level. Solid spinal fusions such as long posterior fusion or anterior-posterior simultaneous fusion are needed in such cases.