Objective To study the stress distribution on different length of pedicle screws under the physiological load by using the three dimensional finite element analysis. Method A threedimension model of the pedicle screw and L1 vertebral body were constructed with the model meshed on the basis of the finite element method. The feature dimension of pedicle screw in the model was set in a specified varied range. Under the physiological load, the stress on every model with different diameter sizes of pedicle screw was analyzed. ResultsThe stress on every bone model decreased with the increase of stress on screw under the axially pullout force as the length of screw ranged from 30mm to 50mm. And the maximum Equivalent Stress (EQV Stress) in the pedicle screw appeared in the central area of the pedicle screw, the maximum Equivalent Stress (EQV Stress) in the cortical bone appeared in both sides of contact surface, the maximum Equivalent Stress (EQV Stress) of the cancellous bone appeared in both sides of contact surface of the top of pedicle screw and cancellous bone. In a certain external load, with the 50mm length of screw, the load that transfers to the cortical bone and cancellous bone is reduced by 43.1% and 42.3%, respectively, while the maximum Equivalent Stress (EQV Stress) of screw was increased 38%. When L≥45mm, the variable stress on all models become stable. Conclusions While the length of screw is in range of 4.0 mm to 6.5 mm, 30～50 mm ,the increase of pedicle screw length could improve the distribution of axial pullout stress on the screws, cortical bone and cancellous bone. As long as the bone mass allowed, the length of pedicle screws should be not less than 45mm in clinical choice.