Objective The aim of this study was to demonstrate the anatomical structures of the transoral approach to the craniovertebral junction. We evaluated the necessary exposure field and the safety of this approach. Methods Surgical operations with the transoral approach were performed on 36 cadaver specimens. The special anatomical structures were measured surrounding the exposure field with priorities given to measurements relating to the vertebral artery (VA). The anatomical relationships between the VA and nerves were observed. Results The exposure field partly covered the vertebral basilar system confluent. The middle clivus to upper C3 vertebral body can be exposed by transoral approach. Cranial nerves and cervical nerves emerged from the caudal of vertebrobasilar artery and circumambulated anterolaterally, and some abnormalities were observed in the intracranial segment of vertebrobasilar artery. The safe field was in an inverted trapezoid shape, of which the widest point was 25.5 ± 4.5 mm to the midline at C1 transverse process level; the narrowest point was 11.2 ± 1.5 mm to the midline at the C2-3 level. Conclusion Because the VA is the landmark of the safe field in this approach, surgeons should be very careful to avoid injuries of the VA and nerves while operating in the intracranial field or at the C2-3 level.