Objective Using the three-dimensional finite element method to analyze the mechanical characteristics of the stress accumulation of the maxillary complex during the expansion until the complete fracture process of the mid-palatal suture and verify the validity of the model. Methods A finite element of the maxillary complex model containing the microimplants was established.The yield strength of the mid-palatal suture was set. the transverse displacement of 0.25 mm was loaded every 5ms as the load until the suture was completely fracture. The CT images of 1 clinical patient before and after the expansion were compared and verified. Result ：0-17ms, the stress is mainly concentrated around the microimplants, the middle of the mid-palatal suture and the zygomatico-maxillary sutures. At the 18ms-60ms cracks began to occur in the mid-palatal suture, and expand forward and backward.. 61-71ms, The rupture path is followed by the posterior part of the mid-palatal suture-the front part-the back part. Conclusion The three-dimensional finite element model of microimplant-assisted expansion based on fracture mechanics was effective and the calculated fracture process results are more accord with clinical practice. A mechanical reference model is provided for more effective research on microimplant-assisted palatal expansion assisted expansion in the future.