To establish theoretical system for classifying parts of air-ground missile, the method for diving air-to-ground missile into different module was proposed. In the paper, the mechanical, electrical, functional and storage correlation of air-to-ground missiles were defined, the method was achieved by using BOM and optimal modularity theory. For clearly expressing use of method, the process of module division was described in the paper. To identify validity of the method, Haierfa air-to-ground missile was selected as an application example. Then, the correlation matrix of missile components was calculated. And the modularity average value and the minimum value were taken as the objective function in optimization and obtained module division of air-to-ground missile. Through the correlation analysis between the internal parts of the divided module, it is shown that the proposed method is useful and expended to modular separation of the other style missile.