Objective To study the mandibular stress distribution and the displacement of the mandibular protraction and to expound the mechanisms of mandibular orthodontic therapy. Methods The model was constructed from class Ⅱ malocclusion in the period of puberty to adulthood. Transverse sections of mandibule were scanned by means of CT. The data was input in the computer. The protrutive relation was set to re established the articulation. The geometry model constructed in CAD software was put into MSC.PATRAN to set up the three dimensional finite element model. Results The results revealed that tensile stress distributed in the posterior edge of the condyle and the molar area and that compressive stress distributed in the inferior edge of the mandible and the gonion area. Conclusion The mandibular protraction could change the stress distribution and treat effectively the skeletal Class Ⅱ malocclusion.