Objective To compare the stress distributions on various tissues in temporomandibular joint (TMJ) before maxillary expansion and with maxillary expander on the maxillary by using the three-dimensional (3D) finite element method. Methods Based on CBCT image data from a patient with maxillary transverse deficiency, the finite element models of TMJ before maxillary expansion and with maxillary expander were reconstructed. The same muscle force and boundary constraint were applied on the two models, and then the von Mises stress, the maximum and the minimum principal stresse on mandibular condylar, joint disc and glenoid fossa were evaluated. Results The von Mises stress of TMJ model before maxillary expansion was mainly distributed in the anterior of mandibular ramus, the anterior of condyle, the middle of articular disc and the top of articular fossa. The pattern of stress distribution on TMJ with maxillary expander was basically consistent with that before maxillary expansion. The TMJ stress was significantly increased, but the stress concentration areas were more evenly distributed. The stress distribution of mandibular condyle and disc moved more laterally and forward. The posterior of mandibular condyle showed a wider distribution of the maximum principal stress. Conclusions The orthopedic force generated by maxillary expander can increase the TMJ stress, stimulate tissue reconstruction of mandibular condyle, and coordinate the condyle and disc. In clinical practice, the patients with maxillary transverse deficiency should be treated with appropriate means of maxillary expansion.