In this paper, the correlative factor of ecological sub-zone, especially the correlative requirement of water use is studied. The role of the specialty of water supply and drainage in an ecological sub-zone is defined. In the light of the knowledge of water supply and drainage specialty, a new design idea for design of outside drainage network in living sub-zone is introduced. There are a lot of suitable conditions of such design to meet the water demands of ecological sub-zone in Chongqing.