Objective To measure the bone mass around the root of mandibular second molar and explore the limiting factors and re-lated factors affecting the distal movement of mandibular second molar.Methods CBCT samples of patients before orthodontic treat-ment were collected.The alveolar bone width,root width,as well as the distance between root and bone cortex were measured at differ-ent levels.Distance between distal edge of crown and anterior edge of mandibular ascending branchwas also measured.The lateral ce-phalic film was generated for routine projection measurement.The correlation of the measurement index and the measurement level,pa-tients age,gender and the shape of mandibular shape were also analyzed.Results Lingual distance and lingual distal distance of the root and bone cortex were the most limiting factors in value.The lingual distance was smaller in levels at the crown side than that in the apical level.There was a positive correlation between the distal distance and the lingual distal distance.There was no significant correla-tion between lingual distal distance and distance between distal edge of crown and anterior edge of mandibular ascending branch.There was no significant correlation between the lingual distal distance and the age,genderor the projection measures related to mandibular morphology.Conclusion The distance between the root and the lingual distal cortical bone may be the limiting factor for distal move-ment of mandibular molars.The distance between the mandibular second molar and the anterior edge of the mandibular ascending branch does not reflect the spatial situation of the distal movement of the molars.There is no significant correlation between the distal movement space of mandibular molars and the age,genderor mandibular morphology of patients.