Objective: To investigate the effects of a Twin-Block appliance combined with slow maxillary expansion (SME) on transverse dental and skeletal parameters in adolescent patients with Angle Class Ⅱ division 1 malocclusion, and to provide a reference for clinical orthodontic practice. Methods: This retrospective study was approved by the Institutional Ethics Committee. A total of 21 adolescents with Class Ⅱ division 1 malocclusion who underwent two-phase treatment with a Twin-Block appliance combined with SME at the Department of Orthodontics, College & Hospital of Stomatology, Guangxi Medical University, in 2021 to 2023 were consecutively enrolled. In the first phase, a functional appliance was used to coordinate the skeletal relationship between the maxilla and mandible by leveraging growth potential. In the second phase, a fixed appliance was employed for fine adjustments of the dental arches based on the specific condition. Cone-beam computed tomography (CBCT) scans were obtained before treatment (T0) and after the first phase of functional correction (T1). Transverse measurements at the first molar region, including molar buccolingual inclination, dental arch width, and basal bone width, were performed using Dolphin 3D Imaging software. Changes between T0 and T1 were statistically analyzed. Results: After the first phase of treatment, the left and right maxillary first molars showed a significant increase in buccal inclination by 5.47° ± 1.38° and 5.35° ± 1.61°, respectively (P<0.001). The arch width in the maxillary first molar region also increased by (2.68 ± 1.14) mm, and the basal bone width increased by (1.14 ± 1.24) mm (all P<0.001). The proportion of skeletal expansion accounted for an average of 42.86%, while dental expansion accounted for 57.14%. No statistically significant changes were observed in any mandibular transverse measurements (all P>0.05). Conclusion In adolescent patients with Angle Class Ⅱ division 1 malocclusion accompanied by maxillary transverse deficiency, Twin-Block appliance combined with SME can effectively expand maxillary dental arch and basal bone width while improving sagittal relationship, thereby correcting transverse discrepancy. The maxillary width changes were predominantly dental.