BACKGROUND: With the advancement of science and technology, invisible aligner without brackets have emerged. However, this new type of digital orthodontic technology is rarely reported on its biomechanics. OBJECTIVE: By the use of rectangular attachments of different sizes and different bonding positions, to obtain different force of canines and the stress supporting of periodontal tissue during the extrusion movement, providing reference for the clinical use of rectangular attachment in the orthodontic treatment of low canine tooth. METHODS: A three-dimensional finite element model of appliance-attachments-canine-periodontium-cancellous bone-cortical bone was established based on the scan data of the isolated canine. The attachments were set in different directions (vertical, horizontal), sizes (3, 4, 5 mm in height) and positions (in the occlusal side of the crown center, in the gingival side of the crown center). The 0.25-mm forced occlusal displacement was loaded to observe the deformation and equivalent stress of the tooth and periodontal membrane. RESULTS AND CONCLUSION: (1) If the size of rectangular attachment increases, the force of the tooth will grow in a certain range and the tooth will have better effect of extrusion with a larger level of tilt motion in other directions. (2) The extrusion effect of the horizontal attachment is better than that of the vertical attachment, which is more difficult to have distal incline, but is prone to lingual incline.