Objective To study the relationship between the retromolar molar space and the depth of the SPEE curve, the degree of crowding of the mandibular dentition in adolescents aged 12~15 years under different sagittal bone shapes, and to evaluate the growth direction and position of the third molar. Methods From January 2020 to December 2023, 182 adolescent patients who were admitted to the Department of Pediatric Dentistry and Orthodontics of the First Affiliated Hospital of Xinjiang Medical University were selected as the research objects. All of them received CBCT and lateral cephalogram examination, including 63 patients of skeletal class Ⅰ, which were included in the skeletal class Ⅰ group; 57 patients of skeletal class Ⅱ, which were included in the skeletal class II group; 62 patients of skeletal class Ⅲ, which were included in the skeletal class Ⅲ group. The retromolar space (RMS), Spee curve depth and mandibular crowding were measured in three groups. The growth direction and position of mandibular third molar were observed. The characteristics of mandibular retromolar space in different sagittal facial types and its correlation with crowding and Spee curve depth were analyzed. Results There were no significant difference between the retromolar space and Spee curves of the left and right molars (P >0.05). There were no statistically significant comparisons of RMS, Spee curve depth and mandibular dentition crowding among different genders (P >0.05). The RMS of skeletal class Ⅱ group was greater than that of skeletal class Ⅰ group (P <0.05), and the RMS of keletal class Ⅲ group was smaller than that of skeletal class Ⅰ group and skeletal class Ⅱ group (P <0.05). The depth of the Spee curve of keletal class Ⅲ group was greater than that of skeletal class Ⅰ group (P <0.05). The crowding degree of mandibular dentition in the skeletal class Ⅲ group was greater than that in skeletal class Ⅰ group and skeletal class Ⅱ group (P <0.05), and the crowding degree of mandibular dentition in skeletal class Ⅰ group was greater than that in skeletal class Ⅱ group (P <0.05). Spearman correlation analysis showed that the depth of Spee curve and the crowding degree of mandibular dentition in the three groups were correlated with the RMS, and the RMS was negatively correlated with the depth of Spee curve and the crowding degree of mandibular dentition (P <0.05). Conclusion There are statistically significant differences in the retromolar space of adolescents with different sagittal bone facial molars. The retromolar space of different sagittal bone types was correlated with the depth of the Spee curve and the congestion of the mandibular dentition.