In Reconfigurable machine tools (RMTs), the reconfiguration only occurs at the mechanical interface system composed of mechanical modules, joining mechanisms, or adapter mechanisms, and their reconfigurability degree depends strongly on the properties of its module interfaces. This paper investigates the performance criteria influencing the reconfigurability of module joint interfaces. An Extenics Element Model (EEM) method is proposed to model the component module of RMTs, module interfaces and the relationship between mechanical modules. Furthermore, the evaluation values are determined based on the Simple Dependent Function (SDF) to evaluate the reconfigurability of different joint interfaces. Finally, two types of joint interfaces between worktable and slide are presented to verify the proposed model and evaluation approaches.