The Human Machine Interface (HMI) is dependent on the Computerized Numerical Control (CNC). As a result, the HMI software is developed according to the interface method of each CNC in duplicate. Even though the same function of HMI software module is developed using the same data item, it must be implemented separately in different development environments. This is because each CNC has a different address system and provides unique data interface method. In this study, we proposed a unified interface system that can standardize and integrate data interface method to support the development of HMI software module applicable to machine tools adopting multi-vendors’ CNCs. To clearly define this work, we developed new parameters, methods, and address system based on the machine state model, composed of data elements of machine tool structure, process, and status, provides the same interface by capsulizing existing CNC interfaces. The proposed interface system is designed to provide an API (Application Programming Interface) in the form of a library. The implementation architecture is designed and details of the operating logic within the detailed components and interfaces are elaborated. The implementation and test results are illustrated to verify an application example of the proposed unified interface system.