Objective To study the biomechanical characteristics of the spine when the growing rod technique is used to treat early-onset scoliosis with different segments fixed. Methods Using the finite element method to establish four spine models of one patient: before operation, T1-L4 segments fixed , T2-L4 segments fixed , and T3-L4 segments fixed, then perform 7 Nm torque control loading and 1 radian angle control loading to analysis the biomechanical characteristics of the adjacent structure of the fixed vertebral body. Results The spinal motion decreased significantly after operation. When the T2-L4 segment was fixed, the spine was most restricted, and the compensatory activity of the adjacent vertebrae increased significantly, and the intervertebral disc stress increased. Conclusion When the upper instrumented segment is T2, the risk of proximal junctional kyphosis and spinal degeneration is the highest, which should be avoided in clinical surgery.