One reason for the delay of open-architecture control system in industry action is that no clear and hierarchical design method has evolved. Different application environment and object should have different open levels, applying platforms, system architectures and integration method. Abstract open control concepts or characters must be converted into specific and practical applying objects and contents of one designing of open-architecture CNC system. Then through analysis of the hierarchy or level of system open character, practical system functional objects to be realized, technical and non-technical issues to be considered, implementation contents and process and the method of component and system validation, a hierarchical design method of open modular architecture CNC system is established.