Objectives To investigate the long-term changes of fixation strength of pedicle screw augmented with calcium phosphate cement in vivo. Methods Twelve pedicle screws were bilaterally planted in to six lumbar vertebrae of each sheep, including 4 non-augmented as control, 4 augmented with calcium phosphate cement (CPC) and 4 augmented with polymethylmethacrylate (PMMA). A total of twelve female adult sheep were operated, and they were randomized into four study periods of 1 day, 4 weeks, 12 weeks and 24 weeks after operation. At the end of every assigned study period, the animals were killed and lumbar vertebrae were harvested. Then axial pull-out tests conducted on the pedicle screws of the isolated vertebrae. The maximum pull-out force (Fmax) and energy absorption value (EAV) of the different screw planting methods at the same time and those of the same screw planting method but at the different study periods were compared by twos, respectively. Results The mean BMD was 0.923±0.082g/cm2 in the group of one day after surgery, 0.910±0.098 g/cm2 group at 4 w, 0.952±0.123 g/cm2 group at 12w and 0.912±0.126 g/cm2 group at 24w. There was no significant difference in the bone mineral density (BMD) of the animals between every two group (p﹥0.05). The mean maximum pull-out force in the CPC group was 893.13±63.91N 24 hours after operation，916.50±109.88N at 4w，1022.04±73.85N at 12w and 1104.91±112.33N at 24w. A positive correlation existed between Fmax and EAV (P﹤0.001). No significant difference in Fmax and EAV was found among the 1-day, 4-week, 12-week and 24- week control group, so were CPC group and PMMA group (P﹥0.28). At the end of every study period, the Fmax and EAV of CPC group were unanimously greater than those of control group (P﹤0.001). And the Fmax and EAV of PMMA group were statistically greater than those of CPC group (P﹤0.003) but an exception, the 24-week EAV (P﹥0.18). Conclusions Results of this research demonstrated that the fixation strength of pedicle screw can be significantly improved by augmentation using the injectable calcium phosphate cement, and the augmentation is not only immediate but maintainable in vivo for quite a long time. CPC may be a favorable substitute of PMMA for the augmentation of pedicle screw.