Objective To introduce a new method to evaluate the flexibility of adolescent idiopathic scoliosis (AIS)-in vivo biomechanical response characteristic curve. Methods A total of 18 patients with Lenke type Ⅰ AIS from Nov. 2017 to Jun. 2018 in our hospital were retrospectively selected; the patients included 16 females and two males, aged from 10 to 17 years old. The initial coronal Cobb angle of the main thoracic curve was 40.00° to 63.00°, averaging at (44.50±6.35)°. The coronal Cobb angle of the main thoracic curve was evaluated by the extracorporeal scoliosis Cobb angle measuring instrument under natural gravity, and then the longitudinal traction force was gradually increased from 0 N to 160 N, with the coronal Cobb angle of the main thoracic curve evaluated every 20 N. Pearson correlation and linear regression analysis were used to analyze the coronal Cobb angle and longitudinal traction for each patient. The regression coefficient (dynamic flexibility index, K) was calculated and the in vivo biomechanical response characteristic curve was drawn. Results The coronal Cobb angle of the main thoracic curve decreased with the increase of longitudinal traction in 18 patients with Lenke type Ⅰ AIS. Pearson correlation regression analysis showed that the coronal Cobb angle of the main thoracic curve in each patient was negatively correlated with the longitudinal traction force (all P<0.05). The K values of the 18 Lenke type Ⅰ AIS patients were -0.239 to -0.098, and K value had no significant correlation with age, height, body weight, body mass index or Risser sign (all P>0.05), but had a negative correlation with the coronal Cobb angle of the initial main thoracic curve (r=-0.704, P=0.001). Conclusion In vivo biomechanical response characteristic curve is an effective method to evaluate the flexibility of scoliosis for Lenke type Ⅰ AIS, and it can dynamically and comprehensively evaluate the overall flexibility.