System characteristics of smart grid such as autonomy， cooperation， hierarchy， evolvement and periodic stability etc.， are analyzed. The concept is put forward that the smart grid is a system of systems composed with the interconnected multiple autonomous systems. Accordingly， a conceptual layered cooperative system of systems model for the smart grid is developed. With this model， the autonomous systems distributed at different layers can be implemented via any one of the totally distributed structure， distributed plus concentrated structure and totally centralized structure，and has the periodic stability characteristic. Each autonomous information system or automation system can set one or more boundary nodes if necessary for communicating between each autonomous system. Moreover， the multiple autonomous systems jointly fulfill the periodic objectives of smart grid. At last， the principle and idea for smart grid development are illustrated under the guidance of the conceptual hierarchical cooperative system of systems model.