Objective To analyze the biomechanical basis of adding-on phenomenon using finite element method to simulate orthodontic surgery for adolescent idiopathic scoliosis (AIS) under different lowest instrumented vertebra (LIV) selections, so as to provide reference and theoretical support for the prevention of adding-on phenomenon after operation. Methods A Lenke type 1BN AIS patient (age 13 years old, body height 152 cm, body weight 51 kg) treated in Department of Spinal Surgery of The First Affiliated Hospital of Naval Medical University (Second Military Medical University) was enrolled. A 3-dimensional finite element model of the patient’s spine was constructed. Then, the AIS orthomorphia under different LIV options was simulated by computer to study the sub-curve compensation and maximum stress of the different unfused intervertebral discs. Results The maximum stress of the unfused intervertebral discs gradually increased from distal to proximal. When the LIV was located at L₁ (the lower end vertebra), the maximum stress of the L_1/2 disc was about 85% higher than that of the L_2/3 disc. When the LIV was located at L₂ and L₃, the maximum stress difference between unfused intervertebral discs was about 20%. Conclusion For Lenke type 1BN AIS patients, LIV below the lower end vertebra can reduce the risk of adding-on phenomenon.