Objective: This study evaluated the feasibility and efficacy of quantitative reduction and fixation to treat basilar invagination (BI) with atlantoaxial dislocation (AAD). Methods: Posterior occipitocervical angle (POCA), occiput–C2 angle (Oc–C2A), clivus-axial angle (CAA), and C2–7 angle (C2–7A) were considered for quantitative reduction. Twelve patients with BI complicated with AAD received posterior interarticular release and individualized cage implantation to restore vertical dislocation. The POCA was adjusted using cantilever technology to further reduce the horizontal dislocation and adjust lower cervical vertebral angle. All patients received a radiological follow-up for ≥12 months. Im-provements in spinal cord function were evaluated using Japanese Orthopedic Association (JOA) score. Results: All the patients received successful quantitative reduction for BI-AAD, and bony fusion was achieved without spinal cord injury after surgery for 12 months. The JOA score was improved significantly to 15. 2 ± 0. 9 twelve months after surgery (p < 0. 01). Radiological follow-up revealed that individualized cage and POCA play vital roles in quantitative correction: (1) distance of the dens above McRae’s line and atlantodens interval were re-stored to normal level, respectively; (2) changes in Oc–C2 angle (ΔOc–C2A), C2–7 angle (ΔC2–7A), clivus-axial angle (ΔCAA), and POCA (ΔPOCA) were all caused by changes in axis tilt. Based on the changes of radiological parameter we deduced the formula for quantitative reduction by linear regression analysis:-ΔPOCA = ΔOc–C2A =-ΔC2–7A = ΔCAA. Conclusion: Quantitative posterior reduction by individualized cage and adjusting ΔPOCA is feasible for treating BI with AAD. © 2020 by the Korean Spinal Neurosurgery Society.