In this essay, the author explained the method of defining functional oning, as one of the classical design methods of city planning from the viewpoint of the juxtaposition and superposition relationships among system elements. Also the au-thor tried to introduce the concept of "superposition" opposite to "juxtaposition"in the process of defining functional area and show the pow-erful promotion brought by "superposition" to de-velop the concepts and methods of traditional defini-tion of functional area and create new juxtaposition relationships through analyzing two design cases.