The patients with cranial defect or abnormality would suffer from the physiological disfunction, and to design a complement proper to the defect part of cranium is the key point in repair. According to the geometric symmetry of healthy cranium, a fast method of reconstructing cranial defects was proposed. First, we used the principal component analysis to determine the initial axis of symmetry, and then combined the moment of inertia to further pinpoint the accurate symmetry-axis linear equation. Finally, we utilized the geometric symmetry to reconstruct the cranial defects. The algorithm reduces the computational complexity of symmetry axis calculation which is always time-consuming, and improves the speed performance while ensuring the accuracy, thereby speeds up the whole reconstructing process. The experiment results proved that this method could effectively calculate symmetry axis of complete cranium CT image, automatically achieve high-precision defects reconstruction without human-computer interaction, and could be used in clinical prosthesis of digital processing production．