Abstract: An analytical method evaluating the connection and sealing performance of non-API casing connections was proposed in this study. First, a mechanical model addressing the sealing and thread meshing of non-API casing connections was developed. According to various failure modes of API connections, such as thread slipping, shear failure, joint rupture, and the elastic-plastic theory, the formulas calculating critical axial loads for non-API casing connections were derived. Second, a set of equations for calculating the contact stress of casing thread were derived according to the deformation compatibility of the meshing thread teeth and the axial deformation of the single thread tooth. Subsequently, the axial deformation of the thread teeth and their load distribution values were obtained. Finally, the performance evaluation result for non-API connections was obtained by comparing the thread teeth's load with the critical axial load. According to the axial deformation calculation results of the casing connection and its influence on the sealing interference fitted value, the sealing performance evaluation results for non-API connections were derived. The evaluation results for non-API connections of three types showed the thread design parameters had significant effects on the connections and sealing performance for non-API casing connections.