Objective　To evaluate the stability of a new posterior dynamic fixation system in atlanto-axial spine.    Methods　Eight fresh human cadaveric specimens (C0~4) were divided randomly into 4 groups: The intact state group (group A), instable state group (group B), posterior atlanto-axial dynamic fixation group (group C) and posterior atlanto-axial pedicle screw fixation group (group D). The rage of motions C1 relative to C2 by loading 1.50 Nm were measured under the six conditions of flexion/extension, left/right lateral bending and left/right rotation．The obtained data was statistically analyzed.     Results    Under the six conditions, there were no significant differences between group C and group A in ROM (P>0.05), but there were significant differences between group C and group D in ROM (P<0.001). There were significant differences between group C and group B, D in stability index under the conditions of flexion/extension, left/right lateral bending and left/right rotation (P<0.05). However, the stability index of group C was mostly similar to the intact state group．Conclusion    In vivo biomechanical studies show that the new posterior atlanto-axial dynamic fixation has unique superiority in the reconstruction of atlanto-axial stability. It may ideally restore the stability of atlanto-axial joint, as well maintain the atlanto-axial range of motion．It is a new surgical choice for treatment of atlanto-axial instability.