Abstract: The fully automatic operation (FAO) system based on GoA4 develops rapidly in the rail transit networks of major cities around the world, the technology of which has been applied widely in the reconstruction of existed lines. In this paper, referring to the successful experiences of the fully automatic transformation of Paris metro Line 1, the difficulties and challenges encountered by FAO system during the reconstruction of existing lines are discussed in depth. On this basis, the necessity and adaptability of the FAO system upgrading during the old line reconstruction in China are analyzed, suggestions related to the development strategy of FAO system based on the longterm and the overall network development are proposed, the technical and management capabilities of the operation departments should be improved at the same time.