Tri-post insulators assume the role of electrical insulation and mechanical support in gas insulated transmission lines (GIL), and are an important component part of AC-GIL. The tri-post insulator is prone to introduce cracking defects during production and operation, which is considered an important causative factor for tri-post insulator breakdown. In this paper, the breakdown characteristics of the tri-post insulators with surface crack defects are studied based on a reduced AC-GIL model. Results show that the electrical and the mechanical damages promote each other during the breakdown propagation of the tri-post insulator with crack defects. The expansion of surface cracks is mainly influenced by the electric field, while the extension of the internal channels of insulators is influenced by the coupling effect of electric field and mechanical stress. Different initial locations and types of crack defects cause different bursting breakdown morphologies of the tri-post insulators. It is hoped that the results of this study will contribute to the analysis of the source of the GIL accident and the optimization of the design of the tri-post insulators.