Objective  To measure the distal space between skeletal class Ⅲ and skeletal class Ⅰ mandibular molars, compare the differences, and to measure the contact situation between the distal root tip of the second mandibular molar and the lingual cortical bone of the mandible, providing a theoretical reference for the distance of mandibular molar distal movement by using cone beam CT(CBCT).  Methods   CBCT imaging data of 60 adult patients who received orthodontic treatment at Chengde Stomatological Hospital from 2020 to 2022 were selected, including 30 patients with skeletal class Ⅲ mean angle and 30 patients with skeletal class Ⅰ mean angle. CBCT image data were exported to DICOM format, then import it into Mimics 20.0 software, and measure the distal space of bilateral mandibular second molars. The contact between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible was measured. Import the measurement results into SPSS 19.0 statistical software to analyze the differences and the contact rate.  Results   There was no statistical difference in the measurement results of the distal space of mandibular molars between genders, and sides, and between mandibular molars with or without wisdom teeth (P>0.05). The distal space of mandibular molars in skeletal class Ⅲ were greater than those in skeletal class I, and the difference was statistically significant (P<0.05). The contact rate between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible was 34.83%. There was no significant difference in the contact rate between the distal root tip of the mandibular second molars and the lingual cortical bone of the mandible between class Ⅲ malocclusion and class I malocclusion(P>0.05).  Conclusion   There is a certain gap in the distal part of the skeletal class Ⅲ mandibular molar. In clinical practice, the molar can be designed to move distally, expanding the dental arch from the sagittal direction to provide a gap for the adduction of the mandibular anterior teeth. However, it is necessary to evaluate the contact between the distal root tip of the second mandibular molar and the lingual bone cortex of the mandible. If the distal root tip of the second mandibular molar contacts the lingual bone cortex of the mandible, the distal movement of the molars should be avoided.