The functions of urban agriculture are objectively or subjectively determined by the demands and resources within urban area, the special space. On the one hand, cities have specific demands for urban agriculture. By virtue of market forces urban agriculture will gradually be molded into agriculture with certain special functions. On the other hand, from the viewpoint of special resources constraints, cities shape urban modern agriculture into special agriculture. The natural development of urban agriculture serves as the foundation for the development of urban agriculture. However, the natural development cannot fully satisfy the demands of cities. These demands will be transformed into the social functions that urban modern agriculture ought to possess.