In this paper, three fundamental principles of city planning under the present system are brought forward: the integration of urban design and city planning, the double attributes and position of the urban design and the correspondence between the urban design and the city planning in levels. Therefore, according to the different circumstances, three functioning ways for the urban design in the practice of plan making are put forward: to put the urban design into the system of city planning, to work out the urban design as a research work and then make use of its achievement in the work of plan making, and to regard the urban design as a specialized work of city planning.