Objective To evaluate the correlation between proximal thoracic curve fusion and postoperative distal adding-on in Lenke type 2A adolescent idiopathic scoliosis (AIS) patients with low left shoulder. Methods The data of 104 patients with Lenke type 2A AIS with low left shoulder who underwent posterior thoracolumbar fusion in our hospital were retrospectively analyzed. All patients were followed up for at least 2 years, and the imaging parameters were measured by X-ray. The patients were divided into ≥T₃ group (upper instrumented vertebra ［UIV］ at T₃ or above T₃) and ＜T₃ group (UIV below T₃) according to the choice of UIV. The imaging parameters, the incidence of distal adding-on and the quality of life (scoliosis research society-22 questionnaire score) were compared between the 2 groups before and after operation and at the last follow-up. Results The average age of the 104 patients was (14.8±3.2) years old, and the average follow-up time was (33.0±9.6) months. There were 71 cases in ≥T₃ group and 33 cases in ＜T₃ group. The preoperative clinical and imaging parameters of the 2 groups were matched (P＞0.05). At the last follow-up, 27 (26.0%) patients had distal adding-on, and its incidence in ≥T₃ group was significantly higher than that in ＜T₃ group (31.0% ［22/71］ vs 15.2% ［5/33］, P＜0.001). Postoperative shoulder imbalance (PSI) occurred in 15 (14.4%) patients, including 4 (12.1%) patients in ＜T₃ group and 11 (15.5%) patientsin ≥T₃ group. There was no significant difference in quality of life scores between the 2 groups (P＞0.05). Multivariate logistic regression analysis showed that Risser grade and UIV selection were independent influencing factors for postoperative distal adding-on in Lenke type 2A AIS patients with low left shoulder (both P＜0.05). Conclusion For patients with Lenke type 2A AIS and lower left shoulder, the choice of UIV is related to the incidence of distal adding-on after operation. For such patients, the fusion of the proximal thoracic curve may increase the risk of distal adding-on.